Define valency of an element. What is the valency of:
Aluminium in aluminium sulfate
Sodium in sodium sulfate

Hint: The valency of an element can be determined easily by the oxidation number of that element, as valency is the ability of the element to combine with other elements to form a compound. The formula of aluminium sulfate is $A{{l}_{2}}{{(S{{O}_{4}})}_{3}}$ and the formula of sodium sulfate is $N{{a}_{2}}S{{O}_{4}}$.

Complete answer:
The ability of the element to combine with other elements to form compounds is known as the valency of the element. The valency of an element can be determined easily by the oxidation number of that element. For example, if the valency of the element is 1, then it will either form +1 oxidation state or -1 oxidation state.
So, we have to find the valency of aluminium in aluminium sulfate. The elements in aluminium sulfate are aluminium, sulfur, and oxygen as its formula is $A{{l}_{2}}{{(S{{O}_{4}})}_{3}}$. Since we know that the oxidation state of sulfate ions is -2, we can calculate the oxidation number or valency of aluminium. Keeping the oxidation of aluminium x, we get:
$2x+3(-2)=0$
$2x-6=0$
$x=+3$
So, the valence or oxidation state of aluminium in aluminium sulfate is +3.
Now, we have to find the valency of sodium in sodium sulfate. The elements in sodium sulfate are sodium, sulfur, and oxygen as its formula is $N{{a}_{2}}S{{O}_{4}}$. Since we know that the oxidation state of sulfate ions is -2, we can calculate the oxidation number or valency of sulfur. Keeping the oxidation of sodium x, we get:
$2x+(-2)=0$
$2x-2=0$
$x=+1$
So, the valence or oxidation state of sodium in sodium sulfate is +1.

Note:
Bonding is a major characteristic of an atom, in which valence term is used. We know that the bonds are formed between two atoms to form a molecule or compound. The bonds are formed between two atoms then the electrons are involved.