Alright, my bright young chemists! Let's embark on an exciting journey into the world of D and F-block elements, specifically focusing on their fascinating oxidation states and the important compounds they form. Think of these elements as the "colorful and versatile" members of the periodic table, always ready to surprise you!

### 1. Understanding Oxidation States: The 'Charge' Game

Before we dive into the D and F-blocks, let's quickly refresh our memory on what an oxidation state (also called oxidation number) actually is. Imagine an atom in a compound. The oxidation state tells us the hypothetical charge that atom would have if all its bonds were *purely ionic*. It's a way to keep track of electron transfer or sharing in chemical reactions.

* Example from S-block: Sodium (Na) in NaCl. Sodium *loses* one electron to become Na⁺. So, its oxidation state is +1. Chlorine (Cl) *gains* one electron to become Cl⁻. Its oxidation state is -1. Simple, right?
* Example from P-block: Oxygen (O) in H₂O. Oxygen is more electronegative than hydrogen, so it *pulls* electrons towards itself, resulting in a -2 oxidation state (each hydrogen is +1).

Atoms try to achieve stability, usually by resembling a noble gas configuration, which involves gaining, losing, or sharing electrons. For S and P block elements, the number of valence electrons is usually fixed, so their oxidation states are often predictable and fewer in number. But wait, that's where our D and F block elements become the "rule-breakers" (or rather, "rule-expanders")!

### 2. D-Block Elements: The Masters of Many Avatars (Variable Oxidation States)

Now, let's talk about the transition elements – the D-block guys. These elements are truly unique because they exhibit variable oxidation states. What does "variable" mean? It means a single transition metal can show *multiple* different oxidation states in different compounds. For instance, iron (Fe) can be +2 in FeCl₂ and +3 in FeCl₃. Manganese (Mn) is a show-off, displaying oxidation states from +2 all the way up to +7!

Why this variability?
This is the core concept for D-block elements. It all boils down to their electronic configuration. Transition metals have electrons in both their (n-1)d orbitals and their ns orbitals (where 'n' is the principal quantum number). And here's the kicker: the energies of these (n-1)d and ns orbitals are very close to each other!

Imagine you have a toolbox. Most elements (S and P block) have tools of very different sizes – it's obvious which tool to pick first. But for D-block elements, it's like having a set of screwdrivers and wrenches that are all almost the same size and weight. You can easily pick out one, two, or even more, depending on the job!

1. Loss of ns electrons: Typically, the first electrons lost are the ones from the outermost ns orbital. This usually results in a +2 oxidation state for most transition metals (e.g., Fe²⁺, Mn²⁺, Cu²⁺).
2. Loss of (n-1)d electrons: Once the ns electrons are gone, the (n-1)d electrons, being very close in energy, can also participate in bonding. Depending on how many d-electrons are involved, the oxidation state can increase. For example, losing one d-electron after the two s-electrons leads to a +3 oxidation state, two d-electrons to +4, and so on.

Let's look at the first transition series (Scandium to Zinc):

Element	Symbol	Electronic Configuration	Common Oxidation States	Most Stable/Predominant States
Scandium	Sc	[Ar] 3d¹ 4s²	+3	+3
Titanium	Ti	[Ar] 3d² 4s²	+2, +3, +4	+4
Vanadium	V	[Ar] 3d³ 4s²	+2, +3, +4, +5	+5
Chromium	Cr	[Ar] 3d⁵ 4s¹	+2, +3, +6	+3, +6
Manganese	Mn	[Ar] 3d⁵ 4s²	+2, +3, +4, +5, +6, +7	+2, +4, +7
Iron	Fe	[Ar] 3d⁶ 4s²	+2, +3	+2, +3
Cobalt	Co	[Ar] 3d⁷ 4s²	+2, +3	+2, +3
Nickel	Ni	[Ar] 3d⁸ 4s²	+2, +3, +4	+2
Copper	Cu	[Ar] 3d¹⁰ 4s¹	+1, +2	+2
Zinc	Zn	[Ar] 3d¹⁰ 4s²	+2	+2

Key observations:
* Scandium only shows +3 (loses all three valence electrons).
* Zinc only shows +2 (loses its two 4s electrons, leaving a stable filled 3d¹⁰).
* Most elements show a common +2 oxidation state (due to loss of 4s electrons).
* The maximum oxidation state generally increases up to Manganese (Mn), where it can lose all its 4s and 3d electrons (2+5=7). After Mn, the stability of higher oxidation states tends to decrease as pairing of d-electrons begins.
* JEE FOCUS: You should be familiar with the common and stable oxidation states for at least the first transition series (Sc to Zn). Understanding *why* they show variable oxidation states is fundamental.

### 3. Important Compounds of D-Block Elements: A Colorful Palette

Because of their variable oxidation states and the availability of vacant d-orbitals, transition metals form an incredibly diverse range of compounds. They are like a versatile artist who can create countless paintings using different colors (oxidation states) and materials (ligands).

What kind of compounds do they form?
1. Oxides: Transition metals form numerous oxides, such as MnO, Mn₂O₃, MnO₂, Mn₂O₇. Notice how manganese can form oxides in different oxidation states. The nature of these oxides changes with the oxidation state:
* Lower oxidation states (e.g., MnO, FeO) are generally basic.
* Intermediate oxidation states (e.g., Cr₂O₃, MnO₂) are often amphoteric (can react as both acid and base).
* Higher oxidation states (e.g., Mn₂O₇, CrO₃) are typically acidic.
2. Halides: Compounds with halogens (F, Cl, Br, I) like FeCl₂, FeCl₃, TiCl₄.
3. Sulfides: Like FeS, NiS.
4. Complex Compounds (Coordination Compounds): This is where transition metals truly shine! They act as central metal ions and form strong bonds with various atoms or molecules called "ligands." These complexes are often highly colored (e.g., [Co(NH₃)₆]³⁺ is yellow-orange, [Ni(H₂O)₆]²⁺ is green) and can be paramagnetic (due to unpaired electrons). We'll dive much deeper into these later!
5. Organometallic Compounds: Compounds containing metal-carbon bonds.

Key characteristics of D-block compounds (overview):
* Color: Most compounds are colored due to d-d electronic transitions.
* Paramagnetism: Many are paramagnetic due to the presence of unpaired d-electrons.
* Catalytic Activity: Many transition metals and their compounds act as excellent catalysts (e.g., Fe in Haber process, Ni in hydrogenation). This is often attributed to their ability to show variable oxidation states and to form intermediate compounds.
* Alloy Formation: They readily form alloys with other metals due to similar atomic sizes.

### 4. F-Block Elements: The Inner Transition Story

Now, let's turn our attention to the F-block elements, also known as the inner transition elements. These are the Lanthanides and Actinides, tucked away at the bottom of the periodic table.

#### a) Lanthanides (4f series):

* Electronic Configuration: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²
* Predominant Oxidation State: The most common and stable oxidation state for almost all lanthanides is +3.
* Why +3? This is because they typically lose their two 6s electrons and one 5d electron (if present) or one 4f electron to achieve a relatively stable +3 state. The 4f electrons are quite deep within the atom and are well shielded by the 5s and 5p orbitals, so they don't participate as readily in bonding compared to d-electrons.
* Other Oxidation States: While +3 is dominant, some lanthanides can show +2 or +4 oxidation states, especially if it leads to an exceptionally stable empty, half-filled, or fully filled 4f orbital (e.g., Eu²⁺ (4f⁷), Yb²⁺ (4f¹⁴), Ce⁴⁺ (4f⁰), Tb⁴⁺ (4f⁷)). These are less common and often less stable.
* Compounds: Lanthanide compounds are typically less colored than transition metal compounds and often resemble each other due to the similar +3 oxidation state.

#### b) Actinides (5f series):

* Electronic Configuration: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²
* Oxidation States: Unlike lanthanides, actinides exhibit a much wider range of oxidation states. The most common state is still +3, but higher oxidation states like +4, +5, +6, and even +7 are observed, especially for the earlier actinides (e.g., U in +6, Np in +7).
* Why more variable? The key difference here is that for actinides, the energies of the 5f, 6d, and 7s orbitals are very, very close and comparable. This makes it easier for a larger number of electrons to participate in bonding, leading to a greater variety of oxidation states.
* Compounds: Actinide compounds are also often colored and can be radioactive (which makes their study more challenging).

JEE FOCUS: For F-block elements, remember the *predominant +3 oxidation state* for lanthanides and the *wider range of oxidation states for actinides* due to comparable 5f, 6d, and 7s orbital energies.

### 5. Stability of Oxidation States: A Quick Peek

The stability of a particular oxidation state depends on various factors:
* Electronegativity of the bonding atom: Higher oxidation states are more stable when combined with highly electronegative elements like oxygen or fluorine (e.g., CrO₃, KMnO₄, OsO₄).
* Crystal field effects (for complexes): In coordination compounds, the ligands influence the stability.
* Electronic configuration stability: Achieving a half-filled (d⁵) or fully filled (d¹⁰) configuration can sometimes stabilize certain oxidation states.
* Acidic/Basic medium: Some oxidation states are stable only in specific pH conditions.

### 6. Summary & Key Takeaways

To wrap it up, remember these crucial points:

* D-Block Elements (Transition Metals):
* Exhibit variable oxidation states due to the involvement of both (n-1)d and ns electrons, which have very similar energies.
* Form a wide variety of colorful, often paramagnetic, and catalytically active compounds.
* Common oxidation states range from +2 up to +7.
* F-Block Elements (Inner Transition Metals):
* Lanthanides: Predominantly exhibit a +3 oxidation state. Other states (+2, +4) are rarer and specific to certain elements.
* Actinides: Show a much wider range of oxidation states (+3 to +7) because the 5f, 6d, and 7s orbitals are of very comparable energies.
* Their chemistry is more complex due to radioactivity and the larger number of available oxidation states for actinides.

Understanding these fundamentals about oxidation states and compound formation is key to unlocking the fascinating and diverse chemistry of D and F block elements. Keep practicing, and you'll master this in no time!

Welcome back, future IITians! Today, we're embarking on a fascinating journey into the world of D and F block elements, focusing specifically on their important compounds and, more importantly, their variable oxidation states. This topic is absolutely crucial for JEE, not just for direct questions but also for understanding the reactivity and properties of these elements in various chemical reactions. So, let's dive deep!

1. Introduction to D-Block Elements and Variable Oxidation States

First things first, what are D-block elements? These are elements where the last electron enters the (n-1)d subshell. They are also known as Transition Elements because their properties generally lie between the highly reactive s-block elements and the covalent p-block elements.

One of their most defining characteristics, and the focus of our discussion, is their ability to exhibit variable oxidation states. Why do they do this?
The core reason lies in their electronic configuration. Transition metals have partially filled (n-1)d orbitals and completely filled or partially filled ns orbitals. The energy difference between these (n-1)d and ns orbitals is very small. This means that both the ns electrons and some of the (n-1)d electrons can participate in chemical bonding.

Consider a general electronic configuration: (n-1)d^1-10 ns^1-2.
When these elements form compounds, they can lose:

1. Only the ns electrons.


2. All the ns electrons plus some (n-1)d electrons.


3. All the ns electrons plus all (n-1)d electrons (for some higher oxidation states).

This flexibility in electron loss leads to the exhibition of multiple stable oxidation states for a single element.

For example, Manganese (Mn) in the 3d series has the configuration [Ar] 3d⁵ 4s². It can exhibit oxidation states from +2 (losing 4s electrons) all the way up to +7 (losing all 4s and 3d electrons), showcasing a remarkable range!

2. General Trends in Oxidation States of D-block Elements

Let's look at some general trends across the D-block:

• Common Minimum Oxidation State: Almost all transition elements show a +2 oxidation state (by losing the two ns electrons). This is very common and often quite stable. Some, like Cu, can show +1.


• Maximum Oxidation State Trend: Across a period (e.g., 3d series), the maximum oxidation state generally increases with the number of unpaired d electrons up to the middle of the series (Manganese, Mn), and then decreases.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

  Element	Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn
  Electronic Configuration	3d¹4s²	3d²4s²	3d³4s²	3d⁵4s¹	3d⁵4s²	3d⁶4s²	3d⁷4s²	3d⁸4s²	3d¹⁰4s¹	3d¹⁰4s²
  Common O.S.	+3	+2, +3, +4	+2, +3, +4, +5	+2, +3, +6	+2, +3, +4, +6, +7	+2, +3	+2, +3	+2, +3	+1, +2	+2
  Maximum O.S.	+3	+4	+5	+6	+7	+3 (+6 in ferrate)	+3	+3 (+4 in some complexes)	+2	+2

  
  This trend is due to the increasing number of d electrons available for bonding, up to 3d⁵, after which pairing of electrons starts, making it harder to involve all d electrons.
  


• Stability of Higher Oxidation States: For elements in the same group, the stability of higher oxidation states increases down the group. For example, Cr(VI) (chromate/dichromate) is a strong oxidizing agent, while Mo(VI) and W(VI) are much more stable and less oxidizing. Similarly, Fe(II) and Fe(III) are common, but Ru(VIII) (RuO₄) and Os(VIII) (OsO₄) are stable. This is attributed to the increasing size of the metal atom, which allows it to accommodate more oxygen atoms and better distribute the charge.


• Stability of Lower Oxidation States: Lower oxidation states (like +2) become less stable down the group for the earlier transition elements (e.g., Ti(+2) is much less stable than Ti(+4)). However, for later transition elements, the +2 state remains quite common.

JEE Focus: Understanding these trends helps predict the reactivity and stability of compounds. For example, knowing that higher oxidation states are more stable for heavier transition metals helps explain why RuO₄ and OsO₄ exist, but FeO₄ does not (or is highly unstable).

3. Important D-block Compounds and Oxidation States (Detailed Examples)

Let's delve into specific elements and their key compounds, highlighting their common oxidation states.

3.1. Chromium (Cr)

Chromium ([Ar] 3d⁵ 4s¹) typically exhibits +2, +3, and +6 oxidation states.

• Cr(II) compounds: Examples include CrCl₂ (chromous chloride). These are generally blue and act as strong reducing agents, easily oxidized to Cr(III).


• Cr(III) compounds: These are the most stable oxidation state for chromium. Examples: Cr₂(SO₄)₃ (chromium(III) sulfate), CrCl₃ (chromium(III) chloride). They are often green or violet in color and are typically found in inert complexes.


• Cr(VI) compounds: This is a very important oxidation state for JEE. Examples include Chromates (CrO₄²⁻, yellow) and Dichromates (Cr₂O₇²⁻, orange). Both are powerful oxidizing agents.
  
  
  
  • Interconversion: Chromates and dichromates are interconvertible based on pH.
    
    In acidic medium, chromate ions (yellow) convert to dichromate ions (orange):
    
    2CrO₄²⁻ (yellow) + 2H⁺ ⇌ Cr₂O₇²⁻ (orange) + H₂O
    
    In alkaline medium, dichromate ions (orange) convert back to chromate ions (yellow):
    
    Cr₂O₇²⁻ (orange) + 2OH⁻ ⇌ 2CrO₄²⁻ (yellow) + H₂O
    
    This equilibrium is a classic example of Le Chatelier's Principle in action.
  
  
  • Potassium Dichromate (K₂Cr₂O₇): A widely used oxidizing agent in volumetric analysis, especially in acidic medium.
    
    Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (E° = +1.33 V)
    
    This shows its strong oxidizing power, reducing itself to the stable Cr³⁺ state.
  
  
  
  

3.2. Manganese (Mn)

Manganese ([Ar] 3d⁵ 4s²) exhibits one of the widest ranges of oxidation states, from +2 to +7, making it a highly versatile element in reactions.

• Mn(II) compounds: The most stable oxidation state for simple Mn²⁺ ions, especially in aqueous solution. Examples: MnSO₄ (manganese(II) sulfate), MnCl₂ (manganese(II) chloride). These are typically pale pink.


• Mn(III) compounds: Relatively less stable than Mn(II), often disproportionate in water. Example: Mn₂O₃.


• Mn(IV) compounds: Manganese dioxide (MnO₂) is the most important compound. It's a black solid, often used as a catalyst (e.g., decomposition of H₂O₂) and as an oxidizing agent. Its stability varies with pH.


• Mn(VI) compounds: Manganate ion (MnO₄²⁻), found in K₂MnO₄ (potassium manganate). It is green and is generally unstable in acidic or neutral solutions, undergoing disproportionation:
  
  3MnO₄²⁻ (green) + 4H⁺ → 2MnO₄⁻ (purple) + MnO₂ (black) + 2H₂O
  
  This reaction is important as it is the intermediate step in preparing KMnO₄ from MnO₂.


• Mn(VII) compounds: Permanganate ion (MnO₄⁻), found in KMnO₄ (potassium permanganate). This is a very powerful oxidizing agent, exhibiting a distinctive intense purple color.
  
  
  
  • Oxidizing power depends on pH:
    
    
    
    • In acidic medium (strongest oxidizing agent): It is reduced to Mn²⁺.
      
      MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (E° = +1.51 V)
    
    
    • In neutral or weakly alkaline medium: It is reduced to MnO₂.
      
      MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻ (E° = +1.23 V)
    
    
    • In strongly alkaline medium: It is reduced to MnO₄²⁻.
      
      MnO₄⁻ + e⁻ → MnO₄²⁻ (E° = +0.56 V)
    
    
    
    

    JEE Focus: Memorize these reduction half-reactions for KMnO₄ in different media, as they are frequently tested in stoichiometry and redox questions.

    
    
  
  
  
  

3.3. Iron (Fe)

Iron ([Ar] 3d⁶ 4s²) commonly exhibits +2 and +3 oxidation states.

• Fe(II) compounds (Ferrous): Examples: FeSO₄·7H₂O (Mohr's salt, ferrous sulfate heptahydrate), FeCl₂. These are typically pale green and are easily oxidized to Fe(III) compounds, especially in the presence of air or oxidizing agents.
  
  4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O


• Fe(III) compounds (Ferric): Examples: FeCl₃ (ferric chloride), Fe₂(SO₄)₃ (ferric sulfate). These are generally more stable than Fe(II) compounds and are often yellow or brown.


• While Fe(VI) exists in ferrates (FeO₄²⁻), they are highly unstable and strong oxidizing agents, less common in typical JEE problems compared to Cr(VI) and Mn(VII).

3.4. Copper (Cu)

Copper ([Ar] 3d¹⁰ 4s¹) primarily shows +1 and +2 oxidation states.

• Cu(I) compounds (Cuprous): Examples: Cu₂O (cuprous oxide, red), CuCl (cuprous chloride, white). These are generally unstable in aqueous solutions and tend to undergo disproportionation:
  
  2Cu⁺ (aq) → Cu²⁺ (aq) + Cu (s)
  
  Cu(I) compounds are stable only when they are insoluble or form stable complexes.


• Cu(II) compounds (Cupric): Examples: CuSO₄·5H₂O (blue vitriol), CuO (cupric oxide, black). This is the most stable oxidation state for copper, especially in aqueous solutions, and responsible for the characteristic blue color of many copper salts.

3.5. Zinc (Zn)

Zinc ([Ar] 3d¹⁰ 4s²) is unique among the 3d series elements because it exhibits only a +2 oxidation state. This is because after losing its two 4s electrons, it achieves a stable, completely filled 3d¹⁰ configuration.
Examples: ZnO (zinc oxide), ZnSO₄ (zinc sulfate), ZnCl₂ (zinc chloride). All are diamagnetic and white (unless complexed with colored ligands).

4. Important F-block Elements: Compounds and Oxidation States (Overview)

The F-block elements are divided into Lanthanoids (4f series) and Actinoids (5f series). Their chemistry is dominated by their unique electronic configurations.

4.1. Lanthanoids (4f series)

Lanthanoids generally exhibit a common and most stable oxidation state of +3.

• Reason for +3: Losing three electrons (two 6s and one 5d or 4f electron) often leads to stable f⁰, f⁷, or f¹⁴ configurations. For instance:
  
  
  
  • Ce (Z=58): [Xe] 4f¹ 5d¹ 6s². Forms Ce³⁺ ([Xe] 4f¹) and Ce⁴⁺ ([Xe] 4f⁰). Ce⁴⁺ is an important strong oxidizing agent, aiming for the noble gas configuration.
  
  
  • Eu (Z=63): [Xe] 4f⁷ 6s². Forms Eu²⁺ ([Xe] 4f⁷) which is a stable configuration, acting as a strong reducing agent. It also forms Eu³⁺.
  
  
  • Yb (Z=70): [Xe] 4f¹⁴ 6s². Forms Yb²⁺ ([Xe] 4f¹⁴), which is also quite stable, acting as a reducing agent. It also forms Yb³⁺.
  
  
  
  


• Other Oxidation States: While +3 is dominant, some lanthanoids show +2 and +4 states, usually to achieve or get closer to empty (f⁰), half-filled (f⁷), or completely filled (f¹⁴) f-subshells. These are exceptions and often have specific redox properties (e.g., Ce⁴⁺ is an oxidizing agent, Eu²⁺ is a reducing agent).


• Common Compounds: Ln₂(O₃) (oxides), LnX₃ (halides), Ln(OH)₃ (hydroxides), where Ln represents a lanthanoid element.

4.2. Actinoids (5f series)

Actinoids exhibit a much wider range of oxidation states than lanthanoids.

• Reason for Variability: The 5f, 6d, and 7s electrons are of very similar energy, making all of them available for bonding. This leads to high variability.


• Common Oxidation States: While +3 is common for later actinoids, earlier actinoids can show much higher states, up to +7.
  
  
  
  • Uranium (U): Displays +3, +4, +5, +6. The +6 state (as in UO₂²⁺, uranyl ion) is very stable.
  
  
  • Neptunium (Np), Plutonium (Pu): Show a maximum of +7, though +3, +4, +5, +6 are also common and stable.
  
  
  • Americium (Am): Shows +3, +4, +5, +6. The +3 state is dominant for later actinoids.
  
  
  
  


• Stability Trends: For earlier actinoids (e.g., Th, Pa, U), higher oxidation states (+4, +5, +6) are more common and stable. For later actinoids (e.g., Am, Cm, Cf), the +3 oxidation state becomes progressively more stable, similar to lanthanoids.


• Compounds: Oxides (ThO₂), halides (UF₆), complex ions (UO₂²⁺).

JEE Focus: For F-block elements, remember the dominant +3 state for lanthanoids and the highly variable nature of actinoids. Be aware of the specific exceptions for lanthanoids (Ce⁴⁺, Eu²⁺, Yb²⁺) due to stable f-configurations, and the high oxidation states for early actinoids.

This detailed overview of oxidation states and important compounds should provide a strong foundation for tackling questions on D and F block elements in JEE. Always remember to connect the observed oxidation states to the electronic configuration and the relative stability of the resulting ions!

Memorizing the common and important oxidation states of D and F block elements, along with their associated compounds, can be challenging due to their variability. Here are some mnemonics and shortcuts to help you remember these key concepts for JEE Main and CBSE exams.

I. D-Block Elements: Oxidation States and Compounds

D-block elements, particularly the 3d series, exhibit variable oxidation states. However, some elements have characteristic or exceptionally stable states that are frequently tested.

1. General Trend for Maximum Oxidation State (3d Series):

• The maximum oxidation state generally increases from Scandium (+3) to Manganese (+7) and then decreases.


• Mnemonic for Elements: "Scared Tiger Vanishes Cruel Man, Fearing Cold Nights, Curtain Zones."
  
  
  
  • Sc: Scandium (+3)
  
  
  • Ti: Titanium (+4)
  
  
  • V: Vanadium (+5)
  
  
  • Cr: Chromium (+6)
  
  
  • Mn: Manganese (+7)
  
  
  • Fe: Iron (+2, +3)
  
  
  • Co: Cobalt (+2, +3)
  
  
  • Ni: Nickel (+2)
  
  
  • Cu: Copper (+1, +2)
  
  
  • Zn: Zinc (+2)
  
  
  
  

2. Distinctive Oxidation States:

Focus on elements with unique or most common stable oxidation states:

• Scandium (Sc): Always shows +3.
  
  Mnemonic: "Scandium is Sc-Triple." (Sc +3)


• Zinc (Zn): Always shows +2.
  
  Mnemonic: "Zn is Zn-Twins." (Zn +2)


• Chromium (Cr): Most common are +3 and +6.
  
  Mnemonic: "Cr-Six for Chrome-Max." (+6 in Chromates/Dichromates)


• Manganese (Mn): Exhibits a wide range, with +2, +4, +7 being very important.
  
  Mnemonic: "Mn shows Many States, highest is Seven." (+7 in Permanganates)


• Copper (Cu): Common states are +1 and +2.
  
  Mnemonic: "Cu is a Couple." (Cu +1 and Cu +2)

3. Important Oxidizing Compounds:

Two crucial compounds from the D-block that act as strong oxidizing agents are Potassium Permanganate (KMnO₄) and Potassium Dichromate (K₂Cr₂O₇).

• In KMnO₄, Mn is in +7 oxidation state.


• In K₂Cr₂O₇, Cr is in +6 oxidation state.


• Mnemonic: "PM DC are strong OA."
  
  
  
  • PM: PerManganate (KMnO₄, Mn is +7)
  
  
  • DC: DiChromate (K₂Cr₂O₇, Cr is +6)
  
  
  • OA: Oxidizing Agents
  
  
  
  

II. F-Block Elements: Oxidation States

Both Lanthanides and Actinides predominantly show a +3 oxidation state, but there are important exceptions.

1. Lanthanides (4f Series):

• Most Common State: +3.


• Exceptions showing +2: Europium (Eu) and Ytterbium (Yb).
  
  Mnemonic: "Europe Yba (yippee!) for +2."


• Exceptions showing +4: Cerium (Ce), Praseodymium (Pr), Terbium (Tb).
  
  Mnemonic: "Certainly Pretty Teabags are +4." (Ce, Pr, Tb)

2. Actinides (5f Series):

• Most Common State: +3.


• Important Higher Oxidation States for Early Actinides:
  
  
  
  • Thorium (Th): +4 (most stable)
  
  
  • Protactinium (Pa): +5 (most stable)
  
  
  • Uranium (U): +6 (most stable)
  
  
  
  
  Mnemonic: "Thor Passed Uranus (+4, +5, +6)."
  


• Later Actinides (Np, Pu, Am, etc.): Exhibit highly variable oxidation states from +3 up to +7.
  
  Mnemonic: "Later Actinides are 'Vary-Actives'." (Highly variable oxidation states)

By using these mnemonics, you can quickly recall the characteristic oxidation states and important compounds of D and F block elements, which is crucial for answering related questions efficiently in exams.

Navigating the various oxidation states and key compounds of d & f-block elements is crucial for JEE Main and board exams. Here are some quick tips to master this topic:

D-Block Elements (Transition Metals)

• Variable Oxidation States: Transition metals exhibit variable oxidation states due to the participation of both (n-1)d and ns electrons in bond formation. The most common and stable oxidation states are those that lead to half-filled or completely filled d-orbitals.


• Highest Oxidation State Trend: For 3d series elements, the highest oxidation state generally increases from Sc (+3) to Mn (+7) and then decreases. It typically corresponds to the sum of ns and (n-1)d electrons.
  
  
  
  • Scandium (Sc): Only +3. (e.g., ScCl₃)
  
  
  • Titanium (Ti): +4 is most stable. (e.g., TiO₂)
  
  
  • Vanadium (V): +5 is most stable. (e.g., V₂O₅)
  
  
  • Chromium (Cr): +3 and +6 are common.
    
    JEE Tip: K₂Cr₂O₇ (dichromate, orange, Cr in +6), CrO₅ (blue, Cr in +6), CrCl₃ (green, Cr in +3).
  
  
  • Manganese (Mn): Exhibits the widest range (+2 to +7).
    
    JEE Tip: KMnO₄ (permanganate, purple, Mn in +7, strong oxidant), MnO₂ (black, Mn in +4), MnSO₄ (pink, Mn in +2).
  
  
  • Iron (Fe): +2 (ferrous) and +3 (ferric) are common.
    
    JEE Tip: FeCl₂ (green), FeCl₃ (yellow/brown). Remember ferric compounds are more stable in air.
  
  
  • Cobalt (Co) & Nickel (Ni): Primarily +2. +3 also exists but is less stable in aqueous solutions.
  
  
  • Copper (Cu): +1 (cuprous) and +2 (cupric) are common.
    
    JEE Tip: Cu²⁺ (blue in solution) is more stable than Cu⁺ (colorless, often disproportionates in aqueous solution). Examples: CuO (black), Cu₂O (red), CuSO₄·5H₂O (blue vitriol).
  
  
  • Zinc (Zn): Only +2. (e.g., ZnO, ZnSO₄)
  
  
  • Silver (Ag): Primarily +1. (e.g., AgNO₃, Ag₂O). AgF₂ exists with Ag in +2.
  
  
  • Gold (Au): +1 and +3. Au(I) in compounds like AuCl, Au(III) in AuCl₃.
  
  
  
  


• Stability Trends:
  
  
  
  • Higher oxidation states are generally more stable with heavier (4d, 5d) transition metals than with 3d metals. For example, Mo and W show stable +6 oxidation states, unlike Cr.
  
  
  • Lower oxidation states are often stabilized by complex formation with $pi$-acceptor ligands (e.g., Ni(CO)₄, Fe(CO)₅).
  
  
  • In oxides and fluorides, metals tend to show higher oxidation states.
  
  
  
  

F-Block Elements (Inner Transition Metals)

• Lanthanoids (Ln):
  
  
  
  • Common Oxidation State: The most characteristic and stable oxidation state for lanthanoids is +3. (e.g., LnCl₃, Ln₂(SO₄)₃).
  
  
  • Exceptions:
    
    
    
    • Cerium (Ce): Shows +4 (e.g., CeO₂, strong oxidizing agent) due to the stability of an empty 4f subshell.
    
    
    • Europium (Eu) & Ytterbium (Yb): Exhibit +2 (e.g., EuCl₂, YbSO₄) due to the stability of half-filled (f⁷) or completely filled (f¹⁴) 4f subshells, respectively. These are good reducing agents.
    
    
    • Pr, Nd, Tb, Dy also show +4 in some oxides but are generally less stable.
    
    
    
    
  
  
  
  


• Actinoids (An):
  
  
  
  • Common Oxidation State: Like lanthanoids, +3 is a common oxidation state.
  
  
  • More Variable Oxidation States: Actinoids exhibit a wider range of oxidation states compared to lanthanoids. This is because 5f, 6d, and 7s energy levels are of comparable energy, allowing for the participation of more electrons.
  
  
  • Examples:
    
    
    
    • Uranium (U): +3, +4, +5, +6 (most stable, e.g., UO₂²⁺ - uranyl ion)
    
    
    • Neptunium (Np), Plutonium (Pu): Show oxidation states from +3 to +7.
    
    
    
    
  
  
  • Stability Trend: The stability of higher oxidation states generally decreases after Plutonium.
  
  
  • Common Mistake: Don't confuse the stability pattern of higher oxidation states for d-block (increasing up to group 7, then decreasing) with that of actinoids (prominent in early actinoids, then decreasing).
  
  
  
  

Focus on identifying the oxidation state of the metal in the given compound and understanding the reasons behind common and exceptional states. Good luck!

Welcome to the 'Intuitive Understanding' section for the important compounds and oxidation states of D and F block elements. This section aims to provide a fundamental grasp of *why* these elements behave the way they do, which is crucial for predicting their properties and reactions.

1. Understanding Variable Oxidation States (D-block)

The most striking characteristic of transition elements (D-block) is their ability to exhibit multiple oxidation states. This phenomenon is largely due to:

• Near-degenerate Energy Levels: The energy levels of the (n-1)d orbitals and the ns orbitals are very close. This means that not only the ns electrons but also some (n-1)d electrons can participate in bond formation.


• Gradual Electron Loss: Unlike main group elements where valence electrons are typically lost all at once, transition metals can lose electrons one by one, leading to various stable oxidation states. For instance, in elements like Sc, Ti, V, Cr, Mn, the highest oxidation state corresponds to the sum of ns and (n-1)d electrons.


• Stability Trends:
  
  
  
  • The +2 oxidation state is very common and often stable, resulting from the loss of the two ns electrons.
  
  
  • Higher oxidation states are generally more stable when the metal is bonded to highly electronegative elements like oxygen or fluorine (e.g., in compounds like KMnO₄ or K₂Cr₂O₇). This is because these elements can effectively stabilize the higher positive charge on the metal.
  
  
  • Lower oxidation states tend to be more basic, while higher oxidation states tend to be more acidic or amphoteric (e.g., CrO is basic, Cr₂O₃ is amphoteric, CrO₃ is acidic).
  
  
  
  

JEE/CBSE Insight: Understanding the reason behind variable oxidation states helps in predicting the redox behavior and the types of compounds formed by transition metals. For instance, knowing that Mn can exist in +2, +4, +6, and +7 states explains its diverse chemistry.

2. Intuition for Important Compound Types (D-block)

Given their variable oxidation states and the availability of vacant d-orbitals, D-block elements form a wide variety of important compounds:

• Oxides: Formed with oxygen, these can range from basic at lower oxidation states (e.g., MnO) to amphoteric (e.g., Cr₂O₃) and acidic at higher oxidation states (e.g., Mn₂O₇, CrO₃).


• Halides: Transition metals form numerous halides (chlorides, bromides, iodides, fluorides) in various oxidation states. For example, FeCl₂, FeCl₃.


• Complex Compounds: This is arguably the most characteristic feature. Due to their small size, high nuclear charge, and availability of vacant d-orbitals to accept lone pairs from ligands, transition metals form a vast number of stable coordination complexes.


• Oxyanions: Metals in high oxidation states, especially Cr and Mn, form stable oxyanions like chromate (CrO₄²⁻), dichromate (Cr₂O₇²⁻), and permanganate (MnO₄⁻), which are important oxidizing agents.

3. Understanding Oxidation States (F-block)

The inner transition elements (F-block) generally exhibit less variable oxidation states compared to D-block elements, but there are important distinctions:

• Lanthanoids (4f series): The most common and stable oxidation state for lanthanoids is +3. This is because the 4f electrons are deeply buried and effectively shielded by the 5s and 5p orbitals, making them less available for bonding. However, some elements exhibit +2 or +4 to achieve stable f⁰, f⁷, or f¹⁴ configurations (e.g., Eu²⁺, Yb²⁺, Ce⁴⁺, Tb⁴⁺).


• Actinoids (5f series): Actinoids show a greater variability in oxidation states than lanthanoids, ranging from +3 to +7 (e.g., U, Np, Pu). This is due to the less effective shielding of 5f electrons and the comparable energies of 5f, 6d, and 7s orbitals, allowing more electrons to participate in bonding. However, +3 is still the most common and stable oxidation state for many actinoids.

JEE/CBSE Insight: Remember the dominance of +3 for both lanthanoids and actinoids, and the key exceptions (Ce⁴⁺, Eu²⁺, Yb²⁺) for lanthanoids and the higher variability for actinoids. This is a frequently tested concept.

By grasping these fundamental reasons, you'll find it much easier to remember and predict the chemical behavior of D and F block elements.

Real World Applications of D & F Block Compounds and Their Oxidation States

The unique properties of d-block (transition metals) and f-block (lanthanides and actinides) elements, particularly their ability to exhibit multiple oxidation states, make their compounds indispensable in a vast array of real-world applications. These applications leverage their catalytic activity, distinctive colors, magnetic properties, and specific reactivities dictated by their electronic configurations and available oxidation states.

Applications of D-Block Element Compounds:

• 
  Catalysis: Many transition metals and their compounds are excellent catalysts, primarily due to their variable oxidation states and ability to form unstable intermediates.
  
  
  
  • Example: Vanadium(V) oxide (V₂O₅) in the Contact Process for sulfuric acid production (V⁺⁵ to V⁺⁴ and back). Iron is used in the Haber process for ammonia synthesis. Nickel is used in hydrogenation reactions. Platinum and Palladium are used in catalytic converters in automobiles (oxidizing CO and unburnt hydrocarbons, reducing NO_x).
  
  
  
  


• 
  Pigments and Colorants: The characteristic colors of transition metal ions (due to d-d transitions and charge transfer phenomena) are widely exploited. The specific color often depends on the oxidation state and ligand environment.
  
  
  
  • Example: Chromium(III) compounds (Cr₂O₃) provide green colors in paints and ceramics. Iron(III) oxide (Fe₂O₃) is used as red ochre. Cobalt compounds produce vibrant blue colors in glass and ceramics. Cadmium sulfide (CdS) provides yellow pigment.
  
  
  
  


• 
  Materials Science and Metallurgy: Transition metals are crucial components in alloys, imparting strength, corrosion resistance, and specific magnetic properties.
  
  
  
  • Example: Chromium, Manganese, Nickel, and Vanadium are alloyed with iron to produce various types of steel with enhanced properties (e.g., stainless steel, tool steel). Titanium alloys are used in aerospace due to their high strength-to-weight ratio.
  
  
  
  


• 
  Electrochemistry and Batteries: Compounds with variable oxidation states are essential in energy storage devices.
  
  
  
  • Example: Lithium-ion batteries often utilize cobalt, nickel, and manganese oxides (e.g., LiCoO₂, LiNiMnCoO₂) as cathode materials, where the transition metal undergoes redox changes during charging and discharging.
  
  
  
  


• 
  Medicine: Some transition metal complexes are used in medical treatments and diagnostics.
  
  
  
  • Example: Cisplatin, a platinum(II) complex, is a widely used anticancer drug. Technetium-99m (^99mTc), a synthetic d-block element, is used extensively in medical imaging (radiopharmaceuticals).
  
  
  
  

Applications of F-Block Element Compounds:

• 
  Permanent Magnets: Lanthanides, particularly neodymium and samarium, form alloys that are the strongest known permanent magnets.
  
  
  
  • Example: Neodymium-iron-boron (Nd₂Fe₁₄B) magnets are vital in motors (electric vehicles, wind turbines), hard drives, and headphones due to their powerful magnetic properties originating from the unpaired f-electrons.
  
  
  
  


• 
  Phosphors and Display Technology: Many lanthanide compounds emit light of specific wavelengths when energized, making them ideal for phosphors.
  
  
  
  • Example: Europium (Eu) and Terbium (Tb) compounds are used as red and green phosphors, respectively, in older CRT displays, fluorescent lamps, and more recently, in some LED applications. Yttrium oxide doped with europium (Y₂O₃:Eu) is a common red phosphor.
  
  
  
  


• 
  Nuclear Energy and Weapons: Actinides, especially uranium and plutonium, are central to nuclear technology due to their radioactivity and fissionable nature.
  
  
  
  • Example: Uranium-235 (²³⁵U) and Plutonium-239 (²³⁹Pu) are used as fuel in nuclear power plants and in nuclear weapons, exploiting their nuclear fission properties.
  
  
  
  


• 
  Medical Imaging: Gadolinium compounds are used as contrast agents in Magnetic Resonance Imaging (MRI).
  
  
  
  • Example: Gadolinium(III) complexes enhance the quality of MRI scans, helping to visualize tissues and organs more clearly due to their paramagnetic properties.
  
  
  
  

Understanding the varied oxidation states and chemical properties of d- and f-block elements is key to appreciating their widespread and critical roles in modern technology and industry. For JEE, focus on associating specific elements/compounds with their primary real-world applications.

Analogies are powerful tools that help simplify complex chemical concepts by relating them to everyday experiences. For the D & F block elements, understanding their unique properties, especially regarding oxidation states and compound formation, can be made easier with the following common analogies.

1. Variable Oxidation States of D-block Elements: The "Swiss Army Knife"

• 
  Analogy: Think of a transition metal atom (D-block element) as a Swiss Army Knife.
  


• 
  Explanation: Just as a Swiss Army knife has multiple tools (blades, screwdrivers, can openers) for various tasks, a transition metal can exhibit various oxidation states. Each "tool" (oxidation state) allows it to participate in different types of chemical reactions, sometimes as an oxidizing agent, sometimes as a reducing agent, and often as a central atom in complex formation. This versatility is crucial for their diverse chemistry.
  


• 
  JEE/CBSE Relevance: This analogy helps grasp why elements like Mn, Cr, and Fe can form so many different compounds with varying properties (e.g., KMnO₄ (Mn⁷⁺) vs. MnO₂ (Mn⁴⁺)).
  

2. Stability of Oxidation States: "The Ladder of Stability"

• 
  Analogy: Imagine the different oxidation states of a transition metal as steps on a ladder.
  


• 
  Explanation: Not all steps on a ladder are equally stable to stand on. Similarly, while a transition metal can access many oxidation states, some are inherently more stable than others. For example, Mn can have +2, +3, +4, +6, +7, but Mn²⁺ (in an aqueous solution) and Mn⁷⁺ (in MnO₄⁻) are quite stable under specific conditions. Reactions often proceed towards achieving these more stable "rungs" on the oxidation state ladder.
  

3. Catalytic Activity: "The Matchmaker or Bridge"

• 
  Analogy: Consider transition metals as matchmakers or a bridge in chemical reactions.
  


• 
  Explanation: Many transition metals and their compounds act as catalysts. They don't get consumed in the reaction but facilitate it. Like a matchmaker bringing two people together who wouldn't ordinarily meet, or a bridge providing an easier path across a river, catalysts provide an alternative reaction pathway with lower activation energy. They achieve this by temporarily changing their oxidation state, forming intermediate compounds, and then regenerating their original state.
  


• 
  JEE/CBSE Relevance: This explains the role of Fe in the Haber process or V₂O₅ in the Contact process.
  

4. Colored Compounds: "The Mood Ring"

• 
  Analogy: The vibrant colors of transition metal compounds are like a Mood Ring.
  


• 
  Explanation: Just as a mood ring changes color based on temperature (or perceived emotion), transition metal compounds exhibit specific colors due to the electronic transitions (d-d transitions) within their partially filled d-orbitals. The energy absorbed to cause these transitions corresponds to specific wavelengths of light, and the complementary color is observed. The specific ligands attached, the oxidation state, and the geometry of the complex can all influence the energy splitting of the d-orbitals, leading to a change in the absorbed wavelength and thus the observed color.
  

5. Lanthanoids vs. Actinoids (Oxidation States): "Conservative vs. Versatile Families"

• 
  Analogy: Think of Lanthanoids as a Conservative Family and Actinoids as a More Versatile Family.
  


• 
  Explanation: Lanthanoids primarily exhibit a stable +3 oxidation state, with only a few showing +2 or +4. They are quite "conservative" in their electron behavior. Actinoids, however, especially the earlier members, show a much wider range of oxidation states (up to +7 for Np, Pu, Am), indicating a greater "versatility" in their chemical behavior due to the comparable energies of 5f, 6d, and 7s orbitals.
  


• 
  JEE/CBSE Relevance: This helps differentiate the characteristic properties of the f-block series.
  

To effectively grasp the concepts of important compounds and oxidation states of d- and f-block elements, a solid foundation in certain fundamental chemistry principles is essential. Revisiting these topics will ensure a deeper and more confident understanding.

Key Prerequisites:

• Atomic Structure & Electronic Configuration:
  
  
  
  • Understanding the Aufbau principle, Hund's rule, and Pauli's exclusion principle for filling electrons.
  
  
  • Knowledge of writing electronic configurations, especially for multi-electron atoms and their ions. This is crucial for d- and f-block elements, as their characteristic properties (variable oxidation states, colour, magnetic behaviour) stem directly from their valence shell (n-1)d and ns electrons, and sometimes (n-2)f electrons.
  
  
  • JEE Focus: Be proficient in writing configurations for ions with varying charges, as this directly impacts the available electrons for bonding and redox.
  
  
  
  


• Periodic Table & Periodicity:
  
  
  
  • Basic trends in atomic/ionic radii, ionization enthalpy, and electronegativity across periods and down groups. These properties influence the stability and nature of compounds formed.
  
  
  • Understanding the position of d- and f-block elements in the periodic table.
  
  
  
  


• Chemical Bonding & Molecular Structure:
  
  
  
  • Basic knowledge of ionic and covalent bonding.
  
  
  • Understanding of valency and how elements combine to form compounds.
  
  
  • An introductory idea of coordinate bonding is helpful, as many d-block compounds form coordination complexes.
  
  
  
  


• Redox Reactions & Oxidation States:
  
  
  
  • Definition and Rules: Clearly understand what an oxidation state is and the standard rules for assigning oxidation states to elements in compounds and ions. This is paramount for this topic, as d-block elements exhibit a wide range of variable oxidation states.
  
  
  • Identifying oxidizing and reducing agents.
  
  
  • JEE Focus: Practise assigning oxidation states in complex inorganic compounds and polyatomic ions, as this is a frequently tested skill.
  
  
  
  


• Nomenclature of Inorganic Compounds:
  
  
  
  • Ability to name simple inorganic compounds and understand common chemical formulas. This will aid in identifying and discussing the 'important compounds' of transition elements.
  
  
  
  

Mastering these foundational concepts will make your journey through the chemistry of d- and f-block elements much smoother and more rewarding for both CBSE and JEE examinations.

Understanding the important compounds and oxidation states of d and f block elements requires a strong foundation in several fundamental chemistry concepts. These related topics provide the necessary background and interconnectedness to grasp the nuances of these unique elements.

Related Topics

• 
  Electronic Configuration:
  
  
  
  • Knowledge of filling of 3d, 4d, 5d, 4f, and 5f orbitals is paramount. The presence of partially filled d or f orbitals is the fundamental reason for variable oxidation states, metallic character, and color.
  
  
  • Specifically, the (n-1)d and ns electrons for transition elements, and (n-2)f, (n-1)d, and ns electrons for inner-transition elements, participate in bonding, leading to variable oxidation states.
  
  
  • JEE Focus: Predicting common and unusual oxidation states often starts with writing the correct electronic configuration.
  
  
  
  


• 
  Periodic Trends:
  
  
  
  • Ionization Enthalpy: Understanding how successive ionization enthalpies influence the stability of various oxidation states (e.g., why +2 is common for 3d series, or why higher oxidation states are more stable with increasing atomic number in 5d series).
  
  
  • Atomic and Ionic Radii: Trends across periods and down groups, and the Lanthanoid Contraction and its effects on the properties of 4d and 5d elements.
  
  
  • Electronegativity: Influences the nature of bonds and the stability of compounds formed with highly electronegative elements like Oxygen and Fluorine.
  
  
  
  


• 
  Redox Reactions and Electrochemistry:
  
  
  
  • The concept of oxidation and reduction is central to understanding the interconversion of different oxidation states (e.g., Cr(VI) to Cr(III), Mn(VII) to Mn(II)).
  
  
  • Balancing redox reactions is a direct application of changes in oxidation states.
  
  
  • Standard Electrode Potentials ($E^circ$): Crucial for predicting the relative stability of various oxidation states and their oxidizing/reducing power (e.g., stability of Mn(II) vs. Mn(III), or why V(V) is a stronger oxidant than Cr(VI) under certain conditions).
  
  
  • JEE Focus: Problems involving titration with KMnO₄ or K₂Cr₂O₇ directly test your understanding of their oxidation states and redox chemistry.
  
  
  
  


• 
  Chemical Bonding:
  
  
  
  • Understanding ionic and covalent bonding helps explain the properties of compounds formed by d and f block elements.
  
  
  • The nature of oxides (acidic, basic, amphoteric) is directly linked to the oxidation state of the metal (e.g., lower oxidation states tend to be more basic, higher more acidic).
  
  
  • CBSE Focus: Basic concepts of bonding types are sufficient.
  
  
  
  


• 
  Coordination Compounds:
  
  
  
  • Many d-block elements form stable coordination complexes. The oxidation state of the central metal ion is a fundamental aspect of coordination chemistry (e.g., [Fe(CN)₆]³⁻, [Co(NH₃)₆]³⁺).
  
  
  • Understanding concepts like Ligand Field Theory and crystal field splitting helps explain the color, magnetic properties, and stability of these complexes, which are directly related to the metal's oxidation state.
  
  
  • JEE Focus: This is a significant intersection. A strong grasp of coordination chemistry greatly aids in understanding the properties of many d-block compounds.
  
  
  
  

By reviewing these foundational topics, students can build a more comprehensive and robust understanding of the d and f block elements, especially concerning their diverse compounds and variable oxidation states.

Key Takeaways: Important Compounds and Oxidation States (D & F Block Elements)

Understanding the common compounds and their associated oxidation states is crucial for D and F block elements, particularly for JEE Main and Advanced. These elements exhibit a wide range of chemical behaviors primarily dictated by their variable oxidation states.

D-Block Elements (Transition Elements)

• Variable Oxidation States: D-block elements are characterized by exhibiting variable oxidation states, primarily due to the availability of both (n-1)d and ns electrons for bonding.


• Common Oxidation States:
  
  
  
  • Most transition elements show a common oxidation state of +2, especially in the first transition series, as the two ns electrons are readily lost.
  
  
  • The highest oxidation state generally increases from Sc to Mn and then decreases. For Mn, it's +7 (e.g., in KMnO₄).
  
  
  • Chromium (Cr): Commonly exhibits +3 (e.g., CrCl₃) and +6 (e.g., K₂Cr₂O₇, CrO₃). Cr(VI) compounds are strong oxidizing agents.
  
  
  • Manganese (Mn): Shows a wide range from +2 (e.g., MnSO₄) to +7 (e.g., KMnO₄). KMnO₄ is a powerful oxidizing agent.
  
  
  • Iron (Fe): Primarily exists in +2 (ferrous, e.g., FeSO₄) and +3 (ferric, e.g., FeCl₃) oxidation states.
  
  
  • Copper (Cu): Exhibits +1 (cuprous, e.g., CuCl) and +2 (cupric, e.g., CuSO₄). Cu(II) is generally more stable in aqueous solutions.
  
  
  • Silver (Ag): Mostly exhibits +1 oxidation state (e.g., AgNO₃, AgCl).
  
  
  • Zinc (Zn): Only shows a +2 oxidation state (e.g., ZnSO₄, ZnO) due to its stable d¹⁰ configuration in Zn²⁺.
  
  
  • Scandium (Sc): Only exhibits a +3 oxidation state.
  
  
  
  


• Stability Trends: Higher oxidation states are more stable in oxides and fluorides, while lower oxidation states are more stable in compounds with halides like Cl, Br, I.


• Important Compounds & Properties:
  
  
  
  • Potassium Dichromate (K₂Cr₂O₇): An orange compound, strong oxidizing agent in acidic medium (Cr is +6). Used in volumetric analysis.
  
  
  • Potassium Permanganate (KMnO₄): A deep purple compound, very strong oxidizing agent in acidic, neutral, or alkaline media (Mn is +7). Used in volumetric analysis.
  
  
  • Copper Sulfate (CuSO₄.5H₂O): Blue vitriol, a common salt where Cu is +2.
  
  
  • Silver Nitrate (AgNO₃): Used in qualitative analysis for halides and in photography.
  
  
  
  

F-Block Elements (Inner Transition Elements)

• Lanthanoids (4f series):
  
  
  
  • Predominant Oxidation State: The most common and stable oxidation state for lanthanoids is +3.
  
  
  • Exceptions: Some lanthanoids show +2 or +4 oxidation states, usually when this leads to stable empty, half-filled, or completely filled f-subshells (f⁰, f⁷, f¹⁴).
    
    
    
    • +2 state: Seen in Eu²⁺ (f⁷) and Yb²⁺ (f¹⁴).
    
    
    • +4 state: Seen in Ce⁴⁺ (f⁰), Pr⁴⁺, Tb⁴⁺. Ce(IV) is a strong oxidizing agent (JEE focus).
    
    
    
    
  
  
  
  


• Actinoids (5f series):
  
  
  
  • Greater Variability: Actinoids exhibit a wider range of oxidation states compared to lanthanoids. This is due to the smaller energy difference between 5f, 6d, and 7s orbitals.
  
  
  • Common Oxidation States: While +3 is also common, higher oxidation states like +4, +5, +6, and +7 are more prevalent, especially in the first half of the series.
    
    
    
    • Uranium (U): Shows +3, +4, +5, +6 (most stable, e.g., UO₂²⁺).
    
    
    • Neptunium (Np), Plutonium (Pu): Show +3, +4, +5, +6, +7.
    
    
    
    
  
  
  • Stability Trend: In the first half of the actinoid series, higher oxidation states (up to +7 for Np) are more stable. In the latter half, +3 is more common and stable, similar to lanthanoids.
  
  
  
  

JEE Tip: Pay special attention to the common and exceptional oxidation states of Cr, Mn, Cu, Ag, Zn in D-block, and Ce, Eu, Yb in Lanthanoids. Understanding the oxidizing/reducing nature associated with these oxidation states is vital for questions involving redox reactions.

CBSE Focus Areas: Important Compounds & Oxidation States (D & F Block Elements)

For CBSE Board Examinations, the topic of "Important compounds and oxidation states" from D & F Block Elements primarily emphasizes understanding the characteristic properties of transition metals and the preparation, properties, and uses of two key compounds: Potassium Permanganate (KMnO₄) and Potassium Dichromate (K₂Cr₂O₇).

1. Variable Oxidation States of Transition Elements

• Definition: Transition metals exhibit variable oxidation states due to the participation of both (n-1)d and ns electrons in bond formation.


• Common Oxidation States: The most common oxidation states for 3d series elements are +2 and +3. Higher oxidation states are generally observed when these elements combine with highly electronegative elements like Oxygen and Fluorine (e.g., Mn in KMnO₄ has +7, Cr in K₂Cr₂O₇ has +6).


• Stability:
  
  
  
  • +2 oxidation state: Formed by the loss of two 4s electrons. Its stability generally increases across the 3d series.
  
  
  • Higher oxidation states: Stability decreases across the period, with the exception of Mn, which shows the highest +7 oxidation state.
  
  
  
  


• Trends to Note (CBSE Perspective):
  
  
  
  • Elements like Sc (+3 only) and Zn (+2 only) exhibit limited oxidation states.
  
  
  • Mn shows the maximum number of oxidation states (+2 to +7).
  
  
  • Elements in the middle of the series show a greater variety of oxidation states.
  
  
  
  

2. Important Compounds for CBSE

A. Potassium Dichromate (K₂Cr₂O₇)

This compound is crucial for board exams. Focus on:

• Preparation: From chromite ore (FeCr₂O₄) involving three main steps:
  
  
  
  1. Conversion of chromite ore to sodium chromate.
  
  
  2. Conversion of sodium chromate to sodium dichromate.
  
  
  3. Conversion of sodium dichromate to potassium dichromate.
  
  
  
  

  Key Reaction: Cr₂O₇²⁻ (orange) <-- (H⁺) --> CrO₄²⁻ (yellow) (pH-dependent equilibrium).

  
  


• Properties:
  
  
  
  • Orange-red crystalline solid.
  
  
  • Powerful oxidizing agent in acidic medium.
    

    Reaction: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

    
    

    Know its reactions with Fe²⁺, I⁻, SO₂/SO₃²⁻, H₂S.

    
    
  
  
  
  


• Uses: Volumetric analysis (oxidizing agent), leather industry, photography.


• Structure: Two tetrahedral CrO₄ units sharing one oxygen atom, resulting in Cr-O-Cr bridge.

B. Potassium Permanganate (KMnO₄)

Equally important for CBSE. Focus on:

• Preparation: From pyrolusite ore (MnO₂).
  
  
  
  1. Fusion of MnO₂ with KOH and an oxidizing agent (like O₂) to form potassium manganate (K₂MnO₄).
  
  
  2. Electrolytic oxidation of potassium manganate (or disproportionation in acidic/neutral medium) to potassium permanganate.
  
  
  
  

  Key Color Change: Green (MnO₄²⁻) to Purple (MnO₄⁻).

  
  


• Properties:
  
  
  
  • Dark purple crystalline solid.
  
  
  • Very strong oxidizing agent in acidic, neutral, and weakly alkaline media.
    
    
    
    • Acidic medium: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (Pale pink/colorless)
    
    
    • Neutral/Weakly alkaline medium: MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻ (Brown precipitate)
    
    
    • Strongly alkaline medium: MnO₄⁻ + e⁻ → MnO₄²⁻ (Green)
    
    
    
    

    Know its reactions with Fe²⁺, C₂O₄²⁻, I⁻, SO₂/SO₃²⁻, H₂S.

    
    
  
  
  
  


• Uses: Volumetric analysis (oxidizing agent), disinfectant, germicide, bleaching wool/cotton.


• Structure: Tetrahedral (Mn is sp³ hybridized).

3. Oxidation States of f-Block Elements (Lanthanoids & Actinoids)

• Lanthanoids: The most common and stable oxidation state is +3. Some may show +2 and +4 due to stability of empty, half-filled, or completely filled f-orbitals (e.g., Eu²⁺, Yb²⁺, Ce⁴⁺, Tb⁴⁺).


• Actinoids: Show a wider range of oxidation states compared to lanthanoids due to comparable energies of 5f, 6d, and 7s orbitals. The +3 state is common, but higher oxidation states like +4, +5, +6, +7 are also observed (e.g., U, Np, Pu).

CBSE Exam Tip:

For KMnO₄ and K₂Cr₂O₇, memorize the balanced redox reactions in different media. Pay attention to the color changes during reactions, as these are frequently asked in MCQs and short answer questions.

Keep practicing the preparation steps and the oxidizing agent behavior of these two important compounds. Good luck!