The
electronegativities of H and N are appreciably different: 2.1 and
3.0, respectively. The three H-N bonds therefore are partly ionic,
or polar, with the electrons held more tightly by N than by H. The
charge in the molecule also is asymmetrical, because of the extra
electron pair at the apex of the pyramid and three H atoms with an
electron deficiency at the base. This means that there actually exists
a small separation of charge on the molecule: The apex is slightly
negative and the base is slightly positive. Thus the molecule is said
to have a tiny dipole.