Total Lone Pairs of Electrons in Melamine Molecule

To determine the total number of lone-pairs of electrons in a molecule of melamine, we analyze its chemical structure:

1. Chemical Formula and Structure: The chemical formula of melamine is $\text{C}_3\text{H}_6\text{N}_6$, systematically known as $1,3,5\text{-triazine-}2,4,6\text{-triamine}$.

   • It consists of a six-membered heterocyclic aromatic ring containing three alternating carbon atoms and three nitrogen atoms (a $1,3,5$-triazine ring).
   • Each of the three carbon atoms in the ring is bonded to an exocyclic amino group ($-\text{NH}_2$).

2. Counting the Lone Pairs of Electrons:

   • Endocyclic Nitrogen Atoms (Ring Nitrogens): There are $3$ nitrogen atoms in the aromatic ring. Each ring nitrogen forms one single bond and one double bond with adjacent carbon atoms (total $3$ shared bonding pairs) and possesses $1$ non-bonding lone pair of electrons in an $sp^2$ hybridized orbital. $\text{Lone pairs from ring nitrogen atoms} = 3 \times 1 = 3$

   • Exocyclic Nitrogen Atoms (Amino Groups): There are $3$ amino ($-\text{NH}_2$) groups attached to the carbons. Each nitrogen atom in an amino group forms three single $\sigma$-bonds (one to a carbon atom and two to hydrogen atoms) and retains $1$ lone pair of electrons. $\text{Lone pairs from amino nitrogen atoms} = 3 \times 1 = 3$

   • Carbon and Hydrogen Atoms: Carbon (valence $4$) and hydrogen (valence $1$) atoms use all their valence electrons in bond formation and contribute zero lone pairs.

3. Total Count: $\text{Total number of lone-pairs} = 3 + 3 = 6$

Thus, the total number of lone-pairs of electrons present in a molecule of melamine is 6.