How are the bonds and lone electron pair arranged around the nitrogen atom in an amine?

In an amine molecule, the nitrogen atom and its substituents form the shape of a somewhat flattened triangular pyramid, with the nitrogen atom positioned at the apex. An unshared electron pair is localized in the region above the nitrogen atom.1

This configuration is dynamic; amines undergo extremely rapid inversion, a process where the molecule effectively turns inside out like an umbrella in the wind, causing the substituents and the nitrogen atom to move in opposite directions. In contrast, quaternary ammonium ions adopt a nearly regular tetrahedral geometry with the nitrogen atom at the center, as the region otherwise occupied by the lone pair is instead filled by a substituent.1