Question

The oxidation state and valency of Al in ${[AlCl{({H_2}O)_5}]^{2 + }}$ :
A.+6 and +3
B.+3 and +6
C.+6, +6
D.+3 and +3

Answer 1

Oxidation state refers to the total number of electrons an atom gains or loses so as to form a chemical bond with other atoms. Whereas valency is related to the number of atoms present in the outermost shell of an atom. Covalency is the number of electron pairs shared by an atom with other atoms.

Complete step by step answer:
-To determine the oxidation state of aluminium, we must know the oxidation states of other atoms present in the complex. We know that the oxidation state of chloride is -1 and since hydrogen is a neutral molecule, its oxidation number is zero.
-As the overall charge on the complex is +2, the sum of the charges of individual atoms will be equal to +2. Thus, the next oxidation state of compound can be written as:
-Oxidation state of aluminium + oxidation state of chlorine + 5(oxidation state of water) = +2
Let the oxidation state of aluminium be x. Then, we have
$x + \left( { - 1} \right) + 5\left( 0 \right) = + 2$
So, $x = + 3$ which means oxidation state of aluminium in ${[AlCl{({H_2}O)_5}]^{2 + }}$ is +3.
-We know that covalency is the number of bonds of ligands attached to that particular atom. -For this complex, we can see that aluminium atom is attached to one chloride ion and five water molecules. So, in total six ligands are attached to the aluminium atom. This tells us that the covalency of aluminium in ${[AlCl{({H_2}O)_5}]^{2 + }}$ is +6.

Hence, the correct option is (B).
Note:
Valency may or may not be equal to the number of valence electrons but covalency always depends on the number of valence electrons. The value of oxidation states can be positive, negative or zero. Oxidation state and valency can be of both, an element as well as for a radical.