Question

What is the valency of phosphate ions in $AlP{O}_{4}$?

Answer 1

Hint: The valency of an element is related to the no. of valence electrons in the outer shell of an atom. It is due to the valency of the elements that they are able to form compounds.

Complete step by step solution:
Valency of an element is the number of hydrogen that can combine with or replace one atom of the element. It is the combining capacity of an element and is the number of electrons an element can gain, lose or share.
If the valence electrons are 4 or less than 4, then the valency is equal to the no. of valence electrons. If the valence electrons are greater than 4, then the valency is equal to 8 minus the number of valence electrons. Some elements may have more than one combining power (or valency), while others just have one.
Coming back to the question, it contains aluminum as the cation and phosphate ion as the anion. The aluminium cation has a +3 valency, so to make the compound neutral in charge, phosphate ions should have -3 charge. The is becomes $P{O}_{4}^{-3}$. Thus, the valency of phosphate ion is -3.

Alternate method:
The valency of the phosphorus =+5 and the valency of oxygen =-2.
In phosphate ions, there are four oxygen atoms. Therefore, the total valency of oxygen = 4x2=8.
And there is one phosphorus atom. Thus, the net valency of phosphate ion = Valency of oxygen + valency of phosphorus.
Therefore, net valency of phosphate ion = -8 + 5 = -3.

Note: The valency of an atom also depends upon its stability. Saying that, means that atoms lose or gain electrons to attain the most stable state. Therefore, the stability of an atom also determines the valency.