Heteronuclear diatomic molecules, electron distribution

For a heteronuclear diatomic molecule, in general the electrons are not shared equally by the two atoms. First, the number of electrons contributed by each atom is not the same. Secondly, the electrons effectively involved in bonding do not usually have a symmetric distribution about the two atoms. As a result, heteronuclear diatomic molecules are usually polar. In the simple case of H-X bonds, the polarity of the HX molecule is directly related to the electronegativity difference of the two atoms. For example, HF is strongly polar in the sense... 

The homonuclear diatomic molecules discussed Section 5.2 are nonpolar molecules. The electron density within the occupied molecular orbitals is evenly distributed over each atom. A discussion of heteronuclear diatomic molecules provides an introduction into how molecular orbital theory treats molecules that are polar, with an unequal distribution of the electron density in the occupied orbitals. 

In the heteronuclear diatomic molecules, the MOs resemble those of Figure 3.1, but the distribution on the two atoms is different because the energy and size of the AOs are different. Indices g and u are excluded since the inversion center is missing. The final charge distribution is polar to some extent. In LiF, it is reasonable to describe the bond as an ionic bond of the type Li+F, formed by first transferring the Li 2s electron to the F 2p subshell, followed by strong interaction between the ions. The measured dipole moment agrees with this picture. In molecules with nearly the same... 

The characteristic feature of heteronuclear diatomic molecules that will be familiar from introductory chemistry is that the electron distribution is not symmetrical between the atoms because it is energetically favorable for a bonding electron pair to be found closer to one atom rather than the other. This imbalance results in a polar bond, which is a covalent bond in which the electron pair is shared unequally by the two atoms. 

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