Question

Write the molecular formulae for the compound: Copper (II) Bromide

Answer 1

Part 1: Molecular formula:

The molecular formula represents the exact number of atoms of an element present in the molecule.

Part 2: Identify the elements and their valencies in Copper (II) bromide:

Copper (II) bromide contains two elements:

1. Copper $\left(\mathrm{Cu}\right)$
2. Bromine $\left(\mathrm{Br}\right)$

Identify the valencies of the elements:

• In Copper (II) bromide, (II) is written after Copper. This suggests that the valency of Copper is $+2$.
• The atomic number of Bromine is $35$ and the electronic configuration is $(2,8,18,7)$.
• The bromine atom gains one electron to complete its octet and forms a bromide ion having one negative charge $\left({\mathrm{Br}}^{-}\right)$.
• Thus, the valency of Bromine is $-1$.

Part 3: Write the molecular formula of Copper (II) bromide:

By the criss-cross method of valency, the valency of Copper is the number of Bromine atoms and the valency of Bromine is the number of copper atoms.

Thus, Copper (II) bromide has one atom of Copper and two atoms of Bromine.

Thus, the molecular formula of Copper (II) bromide is ${\mathrm{CuBr}}_2$.