Valency of Iron (Fe) — Variable Valency, Ferrous, and Ferric States

Variable valency occurs when an element can exhibit more than one combining capacity in different chemical compounds. This is a characteristic feature of transition metals (found in the d-block of the periodic table). They can lose electrons not only from their outermost shell (valence shell) but also from the penultimate (second to last) shell.

Iron has an atomic number of 26. Its electron configuration is [Ar] 3d⁶ 4s².

• The outermost shell is the 4s orbital, which contains 2 electrons.
• The penultimate shell is the 3d orbital, which contains 6 electrons.

When iron reacts, the easiest electrons to remove are the two electrons in the outermost 4s orbital. When it loses these two electrons, it forms the Fe²⁺ ion.

• In this state, its valency is +2.
• Compounds with Fe²⁺ are called Ferrous compounds (e.g., Ferrous Oxide - FeO, Ferrous Sulfate - FeSO₄).

After losing the two 4s electrons, iron is left with a 3d⁶ configuration. According to Hund's Rule of Maximum Multiplicity, exactly half-filled d-orbitals (3d⁵) are highly stable due to symmetry and exchange energy. Therefore, iron often loses one more electron from the 3d orbital to achieve this stable 3d⁵ configuration.

• This forms the Fe³⁺ ion.
• In this state, its valency is +3.
• Compounds with Fe³⁺ are called Ferric compounds (e.g., Ferric Oxide - Fe₂O₃, Ferric Chloride - FeCl₃).

Because the 3d⁵ state is very stable, the +3 valency (Ferric) is often more stable than the +2 valency (Ferrous), especially in the presence of air or oxygen.