Postulates of Valence Bond Theory

Valence bond theory rests on eight postulates. (i) A covalent bond forms when a half-filled valence orbital of one atom overlaps with a half-filled valence orbital of another atom. (ii) The two electrons occupying those half-filled orbitals must have OPPOSITE spins. (iii) During bond formation, as the half-filled orbitals overlap, the electrons' opposite spins neutralise each other, the electron density between the two nuclei increases, the mutual nuclear repulsion correspondingly decreases, and energy is released in the process. (iv) A GREATER extent of orbital overlap gives a STRONGER bond — though complete (100%) overlap never actually occurs, because internuclear repulsion always limits how close the two nuclei can approach. (v) If an atom has more than one unpaired electron, it can form more than one bond — in general, the maximum number of covalent bonds an atom can form equals the number of unpaired (half-filled-orbital) electrons in its valence shell. (vi) The equilibrium distance between two bonded nuclei is where the attractive and repulsive forces exactly balance; at this distance the bonded system's total energy is at a minimum and its stability is at a maximum. (vii) Electrons that are already PAIRED in an atom's valence shell normally cannot take part in bond formation — UNLESS a vacant orbital of only slightly higher energy is available nearby, in which case one electron of the pair can be promoted (unpaired) into that vacant orbital and made available for bonding. (viii) Because the spherical 's' orbital can overlap equally well in …