Build the Ammonia Molecule

Why it matters

Ammonia smells sharp enough to sting your nose, but its shape is surprisingly lopsided. The nitrogen atom holds a secret pair of electrons that pushes the bonds aside.

Predict first

What is the three-dimensional shape of the ammonia molecule?

The lone pair on nitrogen takes up more space than bonding pairs, pushing the three hydrogen atoms down into a pyramid shape.

• A flat triangle with 120° angles
• A trigonal pyramid with a lone pair on top — correct
• A perfect tetrahedron with equal spacing
• A straight line

What you do

Build ammonia (NH₃): nitrogen has 5 outer electrons and needs 3 more; each hydrogen needs 1. Share a pair with each hydrogen — nitrogen keeps its two leftover electrons as a lone pair.

Check yourself

Why is the H-N-H bond angle in ammonia (107°) smaller than the ideal tetrahedral angle (109.5°)?

Lone pairs occupy more volume than bonding pairs, exerting greater repulsive force and squeezing the bond angles.

How many electron pairs (bonding + lone) surround the nitrogen atom in ammonia?

There are 3 bonding pairs (N-H) and 1 lone pair, totaling 4 electron domains.

Which part of the ammonia molecule is responsible for its ability to act as a base?

A base donates an electron pair; the lone pair on nitrogen is available for donation.

• The hydrogen atoms are too large
• The lone pair repels the bonding pairs more strongly — correct
• Nitrogen is less electronegative than hydrogen
• 3
• 4 — correct
• 5
• The hydrogen atoms
• The lone pair on nitrogen — correct
• The N-H bonds