We would expect triethylamine to be polar because of the geometry of the molecule

We would expect triethylamine to be polar because of the geometry of the molecule. The nitrogen has one lone pair of electrons and has three bonded pairs. Therefore, it has ##"sp"^3## hybridized orbitals, which implies a tetrahedral geometry for the electrons. The geometry for the molecule, however, is trigonal pyramidal.

Nitrogen is more electronegative than carbon. This means that the nitrogen is drawing more of the electron of the attached ethyl groups towards it than the ethyl groups are attempting to maintain. The trigonal pyramidal geometry makes the direction of this pull (the dipole) asymmetrical, favoring the "top" of the pyramid where the nitrogen rests. Since the dipole of the molecule is not zero, then it is a polar molecule.