Question

Why is ammonia `NH_3` pyramidal and not trigonal planar (flat)?

Answer 1

It is trigonal pyramidal because of the lone pair of electrons associated with the central nitrogen atom.

Explanation:

The Lewis structure for ammonia looks like this...

![https://useruploads.socratic.org/YlCOcnGfSPGZu24dRCEh_ammonia_lewis.jpg)

You can see the lone pair (nonbonding) of electrons directly above the nitrogen.

The nonbonding pair of electrons pushes away from the bonding pairs producing a trigonal pyramidal shape. If the central atom with no lone pair is bonded to three other atoms the molecule will have a trigonal planar shape.

This video reviews how to draw Lewis structures for molecules.

This video reviews how to determine the shape of molecules.

Hope this helps!