D and F Block Elements Class 12 Notes, IIT JEE & NEET

eSaral ›D and F Block Elements Class 12 Notes, IIT JEE & NEET

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What Are D Block and F Block Elements?

D Block Elements (Transition Metals)

D block elements are those in which the last electron enters the d orbital of the penultimate shell. They occupy Groups 3 to 12 in the periodic table and are called transition metals because they bridge s block (reactive metals) and p block (non-metals) elements.

The four series of d block elements are:

Note for JEE/NEET: Zinc (Zn), Cadmium (Cd), and Mercury (Hg) are d block elements but are NOT transition metals because their d orbitals are completely filled (d¹⁰) in both elemental and common ionic forms. They do not show typical transition metal properties.

F Block Elements (Inner Transition Metals)

F block elements are those in which the last electron enters the f orbital of the pre-penultimate shell. They are placed separately at the bottom of the periodic table and are divided into:

• Lanthanides: Elements 58 (Ce) to 71 (Lu) — 4f series
• Actinides: Elements 90 (Th) to 103 (Lr) — 5f series

💡 Expert Tip by eSaral IIT Faculty: A very common JEE/NEET trap question asks whether Zn is a transition metal. The answer is NO — Zn has a completely filled d¹⁰ configuration in both the atom and its ion (Zn²⁺), so it does not show variable oxidation states, colour, or paramagnetism. Always check ionic configuration, not just atomic configuration.

D and F Block Elements Class 12 Notes

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Electronic Configuration of Transition Metals 

General Configuration

The general electronic configuration of d block elements is:

(n–1)d¹⁻¹⁰ ns⁰⁻²

where n is the outermost shell (principal quantum number).

For the first transition series (Sc to Zn): [Ar] 3d¹⁻¹⁰ 4s⁰⁻²

Important Exceptions to Know for JEE

Two elements in the first transition series deviate from the expected configuration due to the extra stability of half-filled (d⁵) and completely filled (d¹⁰) d orbitals:

💡 Expert Tip by eSaral IIT Faculty: The configurations of Cr and Cu are asked directly in JEE Main and NEET almost every year. Memorise both. The reasoning — extra stability of half-filled and completely-filled d orbitals — must be stated clearly in board exam answers for full marks.

Electronic Configuration of Common Ions

For ionic configurations, always remove ns electrons first, then (n–1)d electrons:

General Properties of D Block Elements

Physical Properties

Transition metals generally have:

• High melting and boiling points (due to strong metallic bonding involving d electrons)
• High density and hardness
• Good electrical and thermal conductivity

Exception: Mercury (Hg) is a liquid at room temperature. Manganese (Mn) has an unusually low melting point among the first series.

Atomic and Ionic Radii Trends

Moving across the first transition series (Sc → Zn):

• Atomic radius decreases initially, then becomes nearly constant from about Cr to Cu
• This is because added d electrons shield the increased nuclear charge effectively — unlike s and p blocks where size decreases steadily
• From Cu to Zn, slight increase due to d¹⁰ repulsion

Ionisation Enthalpy

Ionisation enthalpies of transition metals are higher than s block metals but lower than p block non-metals. The slow increase across the period is due to increasing nuclear charge being partially offset by d electron shielding.

Why Do Transition Metals Show Variable Oxidation States?

Transition metals show variable oxidation states because both ns and (n–1)d electrons can participate in bonding. The energy difference between ns and (n–1)d orbitals is small, making it easy to use different numbers of electrons.

Oxidation State Ranges for Common Transition Metals

Key trends:

• The +2 oxidation state is common across the series (loss of both 4s electrons)
• Mn shows the maximum number of oxidation states (+2 to +7)
• The highest oxidation state increases from Sc (+3) to Mn (+7), then decreases
• Higher oxidation states are stabilised by oxygen and fluorine (electronegative ligands)

Magnetic Properties and Colour of Transition Metal Compounds 

Magnetic Properties

Transition metal compounds are often paramagnetic because of unpaired d electrons.

• Paramagnetism: Substance is attracted to a magnetic field — caused by unpaired electrons
• Diamagnetism: Substance is repelled — all electrons are paired

Magnetic moment (μ) = √n(n+2) BM, where n = number of unpaired electrons

JEE/NEET fact: Mn²⁺ and Fe³⁺ both have d⁵ configuration with 5 unpaired electrons — giving the highest magnetic moment of 5.92 BM among common ions.

Colour of Transition Metal Compounds

Most transition metal compounds are coloured because d electrons can absorb visible light and undergo d-d transitions (electrons jump from lower to higher d energy levels, absorbing part of the visible spectrum — the complementary colour is observed).

Why are Zn²⁺ and Sc³⁺ colourless? Because d⁰ and d¹⁰ configurations have no possibility of d-d transitions — no visible light is absorbed, so the compound appears colourless.

Important Compounds of Transition Metals 

These compounds appear directly in JEE Main, NEET, and board exams every year.

Potassium Permanganate (KMnO₄)

• Mn is in +7 oxidation state
• Strong oxidising agent in both acidic and basic media
• In acidic medium: MnO₄⁻ → Mn²⁺ (colourless) — gain of 5 electrons
• In neutral/alkaline medium: MnO₄⁻ → MnO₂ (brown ppt) — gain of 3 electrons
• Prepared by fusing MnO₂ with KOH in presence of oxidising agent → K₂MnO₄, then electrolytic oxidation → KMnO₄

Potassium Dichromate (K₂Cr₂O₇)

• Cr is in +6 oxidation state
• Strong oxidising agent in acidic medium
• Structure contains two CrO₄ tetrahedra sharing one oxygen
• In acidic solution: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
• Orange in acidic solution, yellow (CrO₄²⁻) in alkaline solution
• Used in volumetric analysis — orange to green colour change indicates end point

F Block Elements: Lanthanides and Actinides 

Lanthanides (4f Series: Ce to Lu, Z = 58–71)

General electronic configuration: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²

Key properties:

• All show +3 as the most common oxidation state
• Also show +2 (Eu, Yb) and +4 (Ce, Tb) in some compounds
• Lanthanide contraction: Steady decrease in atomic/ionic radii from La to Lu due to poor shielding by 4f electrons — each added 4f electron shields nuclear charge poorly, causing a net pull on outer electrons

What Is Lanthanide Contraction and Why Does It Matter?

Lanthanide contraction is the gradual decrease in atomic and ionic radii of lanthanides from La (Z=57) to Lu (Z=71). It occurs because 4f electrons have poor shielding ability — each new 4f electron does not fully shield the increased nuclear charge, causing electrons to be pulled inward.

Consequences of lanthanide contraction:

1. Similar sizes of 4d and 5d transition metals (e.g., Zr and Hf are almost identical in size)
2. Similar properties of Zr/Hf, Nb/Ta, Mo/W — making them difficult to separate
3. Higher density and harder character of 5d metals

Actinides (5f Series: Th to Lr, Z = 90–103)

General electronic configuration: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²

Key differences from Lanthanides:

💡 Expert Tip by eSaral IIT Faculty: The key difference examiners love to test: all actinides are radioactive; only a few lanthanides are. Also, actinides show a wider range of oxidation states than lanthanides because 5f, 6d, and 7s electrons all have similar energies and can all participate in bonding.

Key Differences: D Block vs F Block Elements