Oxidation States and Electron Configurations of Transition Metal Ions HL Only

Specification Reference S3.1.9

Quick Notes

Transition elements show variable oxidation states because:
- Their 4s and 3d electrons are close in energy.
- It requires similar amounts of energy to remove them successively.
Ions are formed by losing electrons from the 4s orbital first, then 3d.
You must be able to deduce electron configurations of ions from Sc (Z = 21) to Zn (Z = 30).

Full Notes

Why Do Transition Metals Show Variable Oxidation States?

For elements in the first row of the d-block (Sc to Zn):

The energy difference between 4s and 3d orbitals is small.
After bonding begins, 4s electrons are lost before 3d because they are at higher energy when filled.
This allows formation of multiple stable ions with different numbers of electrons removed.

Electron Configuration Rules for Ions

Always write the full atomic configuration first.
When forming positive ions:
- Remove 4s electrons first
- Then remove 3d electrons as needed

Example Iron (Fe, Z = 26)

Fe: [Ar] 4s² 3d⁶
Fe²⁺: [Ar] 3d⁶
Fe³⁺: [Ar] 3d⁵

Common Oxidation States in the First Row

Element	Common Ions	Explanation
Sc	Sc³⁺	Loses 3 electrons: 4s² 3d¹ → 0 d-electrons
Ti	Ti³⁺, Ti⁴⁺	Variable loss of 4s and 3d
V	V²⁺, V³⁺, V⁴⁺, V⁵⁺	Many stable states due to small energy differences
Cr	Cr²⁺, Cr³⁺, Cr⁶⁺	Cr³⁺ is most stable (half-filled 3d⁵)
Mn	Mn²⁺, Mn³⁺, Mn⁷⁺	Mn²⁺ is common and stable
Fe	Fe²⁺, Fe³⁺	Both are common, Fe³⁺ in haemoglobin
Co	Co²⁺, Co³⁺	Used in catalysts and complexes
Ni	Ni²⁺	Most common, stable in complexes
Cu	Cu⁺, Cu²⁺	Cu²⁺ is more stable (3d⁹)
Zn	Zn²⁺	Full 3d shell → not a transition element

Summary

Transition elements show variable oxidation states due to small energy differences between 4s and 3d orbitals.
Ions are formed by removing 4s electrons first, then 3d electrons.
Fe, Mn, Cr, and V show especially wide ranges of oxidation states.
Zn and Sc do not fully qualify as transition elements because their common ions lack partially filled d-orbitals.