Question

Classify the following into monovalent, divalent and trivalent ions.
a ) Ammonium
b ) oxide
c ) nitride

Answer 1

A monovalent cation has unit positive charge whereas a monovalent anion has unit negative charge. A divalent cation has two positive charges whereas a divalent anion has two negative charges. A trivalent cation has three positive charges whereas a trivalent anion has three negative charges.

Complete step by step answer:
a ) The chemical formula for the ammonium ion is ${\rm{NH}}_4^ +$. It contains one nitrogen atom, four hydrogen atoms and a positive charge. Ammonium ion can be classified as monovalent ion as it has unit positive charge. Ammonium ion is obtained when a neutral ammonia molecule accepts a proton. Proton has unit positive charge. The lone pair of electrons on nitrogen atoms of ammonia is donated to protons to form ammonium ion. Since nitrogen atoms have donated a pair of electrons, it has gained unit positive charge.
b ) The chemical formula for the oxide ion is ${{\rm{O}}^{2 - }}$. It contains one oxygen atom and two negative charges. Oxide ion can be classified as a divalent ion as it has two negative charges. Oxide ion is obtained when a neutral oxygen atom gains two electrons. Since an oxygen atom has gained a pair of electrons, it has gained two negative charges. In the oxide ion, the difference between the number of electrons and the number of protons is two. These two excess electrons give rise to two negative charges.
c ) The chemical formula for the nitride ion is ${{\rm{N}}^{3 - }}$. It contains one nitrogen atom and three negative charges. Nitride ions can be classified as trivalent ions as it has three negative charges. Nitride ion is obtained when a neutral nitrogen atom gains three electrons. Since nitrogen atoms have gained three electrons, it has gained three negative charges. In the nitride ion, the difference between the number of electrons and the number of protons is three. These three excess electrons give rise to three negative charges.

Note: For a monatomic ion, the valency of cation is equal to the number of electrons lost by a neutral atom during formation of cation. For a monatomic ion, the valency of anion is equal to the number of electrons gained by a neutral atom during formation of anion.