Here's why:

* Central atom: Nitrogen (N) is the central atom.

* Valence electrons: Nitrogen has 5 valence electrons, and each fluorine (F) atom has 7 valence electrons.

* Bonding: Nitrogen forms three single bonds with three fluorine atoms.

* Lone pairs: Nitrogen has one lone pair of electrons.

The lone pair on nitrogen pushes the three fluorine atoms down, resulting in a shape that resembles a pyramid with a triangular base. This is why NF3 is classified as trigonal pyramidal.