Electronic Configurations of the d-Block Elements

NCERT Reference: Chapter 4 – The d- and f- Block Elements, Page 100 (Part I)

Quick Notes

The general electronic configuration for d-block elements is: (n − 1)d^1–10ns^1–2
Series Distribution:
- 3d series: Sc (Z = 21) to Zn (Z = 30)
- 4d series: Y (Z = 39) to Cd (Z = 48)
- 5d series: La (Z = 57) and Hf (Z = 72) to Hg (Z = 80)
- 6d series: Elements after Actinium (Ac)
Exceptions: Some elements have anomalous configurations for added stability:
- Chromium (Z = 24): [Ar] 3d⁵ 4s¹
- Copper (Z = 29): [Ar] 3d¹⁰ 4s¹
- Similarly, Niobium, Molybdenum, Ruthenium, Rhodium, Palladium, Silver, and Platinum show exceptions in the 4d and 5d series.
Stability of Half-filled and Fully-filled Subshells is explained by symmetry and exchange energy.

Full Notes:

The d-block of the periodic table consists of elements in which the last electron enters the (n–1)d orbital. These elements are also called transition elements, as they exhibit properties that are intermediate between those of s-block and p-block elements.

Electronic Configuration of d-Block Elements

he general electronic configuration for d-block elements is:
(n–1)d^1–10 ns^1–2

This means the (n–1) d-orbital can have 1 to 10 electrons, and the ns orbital may have 1 or 2 electrons.

Exceptions

There are exceptions to the general rule due to the small energy difference between (n–1)d and ns orbitals:

Chromium (Cr): Expected: 3d⁴ 4s² → Actual: 3d⁵ 4s¹
Copper (Cu): Expected: 3d⁹ 4s² → Actual: 3d¹⁰ 4s¹

These configurations provide more stability due to half-filled or fully filled d-orbitals.

Special Case – Palladium (Pd): Its configuration is an exception with a completely filled 4d orbital: 4d¹⁰ 5s⁰

Zn, Cd, Hg, Cn (Group 12 Elements)

These have completely filled d¹⁰ configurations in both their ground and common oxidation states: (n–1)d¹⁰ ns²

Because their d-subshells are fully filled and not involved in bonding or oxidation, they are not considered true transition elements.

Properties of d-Orbitals in Transition Elements

The d-orbitals extend to the outer region of the atom more than s or p orbitals.

As a result, they are more affected by surrounding atoms and ligands.

Consequences of d-Electron Configuration

Ions with dⁿ Configuration (n = 1 to 9):

Show similar magnetic and electronic properties.
Common features:
- Multiple oxidation states
- Formation of coloured ions
- Complex formation with ligands
- Catalytic activity
- Paramagnetism due to unpaired electrons

Trends and Similarities

Transition elements show greater similarity across a period (horizontal row) than down a group (vertical column).

These horizontal trends are stronger than in the main group elements.

Summary

The general configuration for d-block elements is (n–1)d^1–10 ns^1–2.
Exceptions such as Cr and Cu arise from half-filled and fully filled d-subshell stability.
Group 12 metals have d¹⁰ in common oxidation states and are not true transition elements.
d-electrons underpin variable oxidation states, coloured ions, complex formation and catalysis.