Question

What is the valency of Phosphate ions in $AlPO_4$ ?

Answer 1

Hint: The concept of valency is related to number of valence electrons in an atom. Thus, valency can be loss of electrons, gain of electrons, or sharing of electrons by an atom.
Here we have to calculate the valency of phosphate Ion. This valency is responsible for formation of compounds.

Complete answer:
Valency of elements is the combining capacity of that element by gain of electron, loss of electron or sharing of electrons. Here in the compound aluminium phosphate, we know that there are two ions. A positive ion is aluminium ion, which has a charge of +3, as there are three electrons in the aluminium atom, so the 3 electrons can be lost to gain stability.
Let us consider the valency of phosphate ions to be ‘x’.
Now, since the overall compound has no charge, we can write.
Charge on aluminium ion + Charge on phosphate ion = 0
Substitute charge of aluminium and phosphate.
3 + x = 0
Therefore, x= - 3
Now we came to know the charge on phosphate ions is -3, thus the valency is 3, which is the numerical value for this case, as it is the loss of electrons.

Thus, valency of phosphate is calculated as 3.

Additional Information: We know that in phosphate ion, Phosphorus has valency of +5, while oxygen has valency of -2, but there are 4 atoms of oxygen. Thus, we can do calculation of valency of phosphate ions.
Total charge = Total charge of Phosphorus + Total charge of oxygen
Here we have to multiply 4 with -2, as there are 4 oxygen atoms.
= 5 + 4 x (-2)
= -3
total charge on phosphate ion = -3
Thus charge is -3, the numerical value is the valency of phosphate ions.
Thus valency is 3.

Note: There are different ways of finding valency for cation, anions and sharing of electrons.
For cation, Valency = Numerical value of charge on cation.
For anion, Valency = Numerical value of charge on anion.
For covalent compounds, Valency = Number of electrons shared.