Question

What is the oxidation number of $H$ in $Al{H_3}$?

Answer 1

Hint: The oxidation number is defined as the number of electrons that atoms in a molecule can share, lose or gain while forming chemical bonds with other atoms of different elements. It can also be defined as when an atom acquires a charge after forming bonds with other heteroatoms.

Complete answer:
Let the oxidation state of hydrogen be x. Here both the atoms that are Aluminum and Hydrogen are electron donating atoms so they will acquire positive oxidation states or numbers. But here hydrogen behaves as an electronegative element or electron gain element because if we assign $ + 1$ oxidation number to hydrogen then the oxidation number of aluminum comes out to be $ - 3$ which is not possible as aluminum is an electropositive element. So therefore aluminum carries a charge of $ + 3$ in this molecule.
Now, $x + 3 = 0$
So, $x = - 3$
It means that one hydrogen atom has an oxidation number of $ - 1$ here. Here, we put the sum of these two oxidation numbers equal to zero because the whole molecule should always be neutral in nature.

Note:
While calculating the oxidation number we should keep two things in mind and that is the atom should make bonds with heteroatoms and the second one is that the atoms should form ionic bonds by gaining or losing an electron instead of forming covalent bonds that is sharing of electrons. The atoms should not depend upon the actual nature of bonding while finding their particular oxidation number.