Question

Write down the formulae of sodium sulfide.

Answer 1

Sodium sulfide is a compound of sodium cation and sulfide anion. The formula represents the number of each ion present in a compound.

Complete answer:
A chemical formula is an expression of elements which indicates the relative proportions of the specific elements present in a definite ratio. For example, the compound water is a compound which is composed of hydrogen and oxygen in a $2:1$ ratio. Thus the chemical formula of water is ${H_2}O$.
A chemical compound is a substance which is composed of different type of atoms which are bonded together by chemical bonds. A molecule is different from a compound as it contains only one type of atoms.
The given compound is sodium sulfide. It consists of two elements of which one is sodium and the other is sulfur. Sodium is an element in the periodic table with atomic number $11$. Its electronic configuration is $\left[ {Ne} \right]3{s^1}$. The valence shell of sodium is $3$ and it contains a single electron for donation or making bonds and shows a valency of $1$.
The sodium metal belongs to alkali earth metals. It is very electropositive in nature and has the tendency to lose the valence shell electron readily. Thus it forms the cation of the given compound.
Sulfur is an element in the periodic table with atomic number $16$. Its electronic configuration is $\left[ {Ne} \right]3{s^2}3{p^4}$. The valence shell of sulfur is $3$ and it contains $6$ electrons of which two in $3s$ and four in $3p$ is present. To attain the octet it needs two electrons. So it shows a valency of $2$. The sulfur forms the anion part of the compound.
The valencies of both the cation and the anion are satisfied by each other. To make a neutral molecule two sodium atoms are required for making bonds with a single sulfur atom. Thus both the sodium atoms and the sulfur atom complete the octet. Hence the formula of sodium sulfide is $N{a_2}S$.

Note:
The octet is essential for all the elements to make bonds with other elements. It makes the molecule stable by lowering the energy.