Question

The oxide of aluminium has a chemical formula $A{l_2}{O_3}$. State the valency of $Al$.

Answer 1

The oxide of aluminium consists of aluminium cation and oxide anion. A chemical formula represents the exact proportion of the elements in which they are present in a compound. The valency is defined as the combining power of an element which is used to form a chemical compound.

Complete step by step answer:
Aluminium is an element in the periodic table. The atomic number of aluminium is $13$. The electronic configuration of aluminium is:
$Al = 1{s^2}2{s^2}2{p^6}3{s^2}3{p^1}$
The outermost shell of aluminium is $3$. This corresponds to the principal quantum number $n = 3$. This shell is also labeled as M shell. So for aluminium to make compounds three electrons, two from s orbital and one from p orbital is available for donation.
Valency is equal to the number of electrons in the outermost shell which can be added or removed to form compounds and reach octet. Now the chemical formula of aluminum oxide is $A{l_2}{O_3}$. In terms of ions, the formula of aluminum oxide can be written as:
$A{l_2}{O_3} = A{l^{3 + }}{O^{2 - }}$
The power of cation is written as the subscript of anion and the power of anion is written as the subscript of cation. The power of aluminium is $3 +$ and the power of oxygen is $2 -$.
Thus the valency of aluminium in $A{l_2}{O_3}$ is $3$.

Note: In a similar way the valency of oxygen can be determined. The oxygen atom has atomic number and has electronic configuration $1{s^2}2{s^2}2{p^4}$. So the outermost shell is $2$ and it needs two electrons to attain its octet. Thus the valency of oxygen atom in $A{l_2}{O_3}$ is $2$.