Question

Which of the following pairs of elements are likely to form an ionic or a molecular compound? Why?

A. Calcium and Bromine

B. Iron and Oxygen

Answer 1

Fajan's Rule predict whether a bond will be covalent or ionic depending on the charge and relative sizes of the cation and anion.

Fajans’ Rule postulates the following conditions for covalent componds:

1. Size of the ion: Covalent character of the bond increases as size of cation decreases and size of the anion increases

2. The charge of Cation: Covalent character increases with increased charge over the cation

3. Electronic configuration: The typical (n-1)dⁿ ns^o types of electronic configuration in transition elements form better covalent compounds than the cation with ns² np⁶ electronic configuration in alkali or alkaline earth metals

In the first case;

Calcium is an s-block element and Bromine is a p-block element.

Electronic configuration of Ca = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² and Ca²⁺ = 1s² 2s² 2p⁶ 3s² 3p⁶

Electronic configuration of Br = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵ and Br⁻ = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶

The size of Ca²⁺ is not small and it does not have higher charge over it. The size of Br⁻ is small due to presence of 10 d electrons and it also does not have higher charge over it.

So Calcium and Bromine forms ionic compound Calcium bromide.

In the second case:

Iron is a transition element i.e. d-block element and Oxygen is a p-block element.

Electronic configuration of Fe = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ and Fe²⁺ = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s⁰ 3d⁶

Electronic configuration of O = 1s² 2s² 2p⁴ and O²⁻ = 1s² 2s² 2p⁶

Now size of the cation is smaller due to presence of d electrons and their poor shielding. Also size of anion here is bigger. Thus charge by size ratio increases for cation and decreases for anion giving rise to formation of a covalent compound.

So Iron and Oxygen forms covalent compound Iron oxide.