Understanding Variable Oxidation Numbers

An oxidation number (or oxidation state) represents the hypothetical charge an atom would have if all bonds to atoms of different elements were 100% ionic. It indicates the degree of oxidation (loss of electrons) or reduction (gain of electrons) of an atom.

Variable oxidation numbers mean that an element can exist in multiple oxidation states in different chemical compounds. This is a common characteristic of transition metals, which have partially filled d-orbitals that allow them to lose varying numbers of electrons. Elements from the s-block (like alkali metals and alkaline earth metals) typically exhibit only one common oxidation state.

Analyzing Element Oxidation States

Let's examine the common oxidation states for the elements provided in the options:

• Sodium (Na): Sodium is an alkali metal (Group 1). It has one valence electron in its outermost shell (\(3s^1\)). To achieve a stable electron configuration like Neon, it readily loses this electron, resulting in a charge of +1. Therefore, sodium almost exclusively exhibits an oxidation number of +1 in its compounds.
• Calcium (Ca): Calcium is an alkaline earth metal (Group 2). It has two valence electrons in its outermost shell (\(4s^2\)). To attain a stable configuration like Argon, it loses these two electrons, resulting in a charge of +2. Thus, calcium typically shows an oxidation number of +2.
• Iron (Fe): Iron is a transition metal (in the d-block). Its electron configuration is [Ar] \(3d^6 4s^2\). Transition metals are known for their ability to display multiple oxidation states because they can lose electrons from both their outermost s-orbital and their inner d-orbitals. Iron commonly exhibits two oxidation states:
  • +2 (Ferrous state): When iron loses its two \(4s\) electrons, forming the \(Fe^{2+}\) ion (e.g., in ferrous sulfate, \(FeSO_4\)).
  • +3 (Ferric state): When iron loses the two \(4s\) electrons and one \(3d\) electron, forming the \(Fe^{3+}\) ion (e.g., in ferric oxide, \(Fe_2O_3\)).
  Because Iron can exist in more than one oxidation state (+2 and +3), it shows variable oxidation numbers.
• Lithium (Li): Lithium is also an alkali metal (Group 1). Like sodium, it has one valence electron (\(2s^1\)) and readily loses it to achieve the stable electron configuration of Helium. Consequently, lithium primarily exhibits an oxidation number of +1.

Identifying Elements with Variable Oxidation Numbers

Based on the analysis, Sodium, Calcium, and Lithium are predictable elements belonging to Group 1 and Group 2, respectively, and they typically display only a single, fixed oxidation number (+1 for Na and Li, +2 for Ca). Iron, being a transition metal, possesses the characteristic ability to exhibit multiple oxidation states. Therefore, Iron is the element among the choices that exhibits variable oxidation numbers.