Here's why:

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

* Valence electrons: Nitrogen has 5 valence electrons, and each bromine atom has 7 valence electrons.

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

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

* VSEPR theory: According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, the electron pairs (bonding and lone pairs) around the central atom will arrange themselves to minimize repulsion. In this case, the four electron pairs (three bonding pairs and one lone pair) arrange themselves in a tetrahedral shape.

* Molecular geometry: Since the lone pair is not visible, the molecular geometry is described by the positions of the bromine atoms only, giving a trigonal pyramidal shape.

Therefore, nitrogen tribromide has a trigonal pyramidal molecular geometry with bond angles slightly less than 109.5° due to the lone pair's influence.