In case of trivalent N, the molecules are pyraMIDAL in nature.one of the orbitals is occupied by the lone pair of electrons.now, due to vibration in the molecule along the axis perpendicular to the bond axis ,the molecules are inverted and the two forms remain in equilibrium state .these two forms are non superimposablemirror images(showing enantiomerism) --- this phenomenon of trivalent n compounds are known as INVERTOMERISM