Elements like iron (Fe) can have more than one valence number because they can form multiple ions with different charges. This phenomenon is known as variable valency or variable oxidation states.
The valence number or valence state of an element indicates the number of electrons that an atom of that element can gain, lose, or share when it forms chemical compounds. It represents the charge on the ion formed by the element.
In the case of iron, it can form two common ions with different charges: Fe2+ and Fe3+. The Fe2+ ion is formed when an iron atom loses two electrons from its outermost shell, resulting in a 2+ charge. The Fe3+ ion, on the other hand, is formed when an iron atom loses three electrons, resulting in a 3+ charge.
The ability of iron to exhibit variable valency arises from its electronic configuration. The electron configuration of iron is [Ar] 3d6 4s2, meaning it has two electrons in its outermost 4s orbital and six electrons in its 3d orbital. In certain chemical reactions, iron can lose either the 4s electrons or the 3d electrons to achieve a stable configuration.
The specific valence state that iron adopts in a given compound depends on the nature of the compound and the other elements it interacts with. Iron can exhibit different valence states depending on factors such as the oxidation-reduction (redox) conditions, the presence of ligands, and the overall electronic structure of the compound.
For example, in iron(II) compounds (Fe2+), iron has a +2 oxidation state and forms compounds like iron(II) chloride (FeCl2). In iron(III) compounds (Fe3+), iron has a +3 oxidation state and forms compounds like iron(III) oxide (Fe2O3).
The variable valency of elements like iron allows them to participate in a wide range of chemical reactions and form diverse compounds with different properties. It also plays a crucial role in the biological functions of these elements within living organisms.