Concept: Counting Lone Pairs in Melamine

Melamine (C₃H₆N₆) is a nitrogen-rich compound. To find the total number of lone pairs of electrons, we need to determine the number of non-bonding valence electrons. This is done by first drawing the Lewis structure to understand the bonding and electron distribution.

Step 1: Determine the Total Number of Valence Electrons

First, calculate the total valence electrons from all atoms in the molecule.

• Carbon (C) has 4 valence electrons. There are 3 carbon atoms: $3\times 4=12$
• Hydrogen (H) has 1 valence electron. There are 6 hydrogen atoms: $6\times 1=6$
• Nitrogen (N) has 5 valence electrons. There are 6 nitrogen atoms: $6\times 5=30$

Total valence electrons = $12+6+30=48$

Step 2: Draw the Skeletal Structure and Form Bonds

The common structure of melamine is a triazine ring, which is a hexagon with three carbon and three nitrogen atoms alternating. Each carbon atom is bonded to an amino group (-NH₂).

Let's count the number of bonds (each bond uses 2 electrons):

• Ring bonds: The 6-membered ring has 6 bonds. Electrons used = $6\times 2=12$
• C-N bonds to amino groups: Each of the 3 carbon atoms is bonded to a nitrogen of an -NH₂ group. This is 3 bonds. Electrons used = $3\times 2=6$
• N-H bonds: Each amino group has 2 N-H bonds. With 3 amino groups, that's 6 N-H bonds. Electrons used = $6\times 2=12$

Total electrons used in bonding = $12+6+12=30$

Step 3: Calculate the Number of Non-Bonding Electrons

Subtract the bonding electrons from the total valence electrons to find the non-bonding (lone pair) electrons.

Non-bonding electrons = Total valence electrons - Bonding electrons

Non-bonding electrons = $48-30=18$

Step 4: Find the Number of Lone Pairs

A lone pair consists of 2 electrons. Therefore, we divide the number of non-bonding electrons by 2.

Number of lone pairs = Non-bonding electrons / 2

Number of lone pairs = $18/2=9$

Final Answer

The total number of lone-pairs of electrons in melamine is $9$.

Related Topics

• Lewis Structures: Diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist.
• Valence Electrons: The electrons in the outermost shell of an atom that are involved in bonding.
• Formal Charge: A concept used to determine the most likely Lewis structure by calculating the charge assigned to an atom in a molecule.
• VSEPR Theory: Valence Shell Electron Pair Repulsion theory, which predicts the geometry of molecules based on the repulsion between electron pairs.

Key Formulae and Theory

• Total Valence Electrons: Sum of valence electrons from all atoms in the molecule.
• Bonding Electrons: Total electrons used in forming bonds (Number of bonds × 2).
• Non-Bonding Electrons: Total Valence Electrons - Bonding Electrons.
• Number of Lone Pairs: Non-Bonding Electrons / 2.
• Octet Rule: Atoms tend to form bonds to have 8 electrons in their valence shell, achieving a stable noble gas configuration. Hydrogen is an exception, seeking 2 electrons (duet rule).