Bonding between atoms

One way to picture the bonding between atoms is to use the cocept of molecular orbital theory. MO theory considers the electron wavefunctions of the individual atoms combining to form molecular wave functions(or molecular orbitals as they are known). These orbitals, which are now delocalized over the whole molecule, are then occupied by all the available electrons from all the constituent atoms in the molecule. Molecular orbitals are really only formed by the wavefunctions of the electrons in the outermost shells (the valence electrons) ie. Those which significantly overlap in space as atoms become progressively closer together; the inner electrons remain in what are essentially atomic orbitals bound to the individual atoms.


GIANT MOLECULAR SOLIDS

When atoms come into close proximity with other atoms in a solid, most of the electrons remain localized and may be considered to remain associated with a particular atom. However, some outer electrons will become involved in bonding with neighbouring atoms. Upon bonding the atomic energy level diagram is modified. Briefly, the well defined outer electron states of the atom overlap with those on neighbouring atoms and become broadened into energy bands. One convenient way of picturing this is to envisage the solid as a large molecule.

For a simple diatomic molecule the two outermost atomic orbitals AOs overlap to produce two molecular orbitals which can be viewed as a linear combination of the two constituent atomic orbitals. As before, the bonding MO is formed from the in-phase overlap of the AOs and is lower in energy than the corresponding AOs, whereas the other MO, formed from the out of phase overlap, is higher in energy than corresponding AOs and is termed an antibonding MO. progressively increasing the length of the molecular chain increases the total number of MOs, and gradually these overlap to form bands of allowed energy levels which are separated by forbidden energy regions (band gaps). These band gaps may be thought of as arising from the original energy gaps between the various atomic orbitals of the isolated atoms.



Kelsall R; Nanoscale science and technology; WILEY (Blackwell) 2006