Electrons also can pull a molecule apart, an idea that could not be accounted for with the old Lewis theory. The other way of combining the 1s wave functions, with opposite signs, leads to an MO in which the greatest electron density lies on the outside of the nuclei, away from the bond. In such a molecular orbital, there is zero probability of finding electrons on a plane halfway between the nuclei (see previous page). Electrons in this (1s - Is) orbital not only fail to shield the nuclei from one another, they actively pull the molecule apart from each end. Such an MO is called an antibonding orbital because it is less stable (higher energy) than the orbitals of the separated atoms. Some terminology must be introduced at this point to make the subsequent discussion easier. If a molecular orbital is completely symmetrical about the line connecting the nuclei, it is called a s- (sigma) MO. Bonding MO's are further indicated by a small superscript b, and antibonding orbitals by a small asterisk. (Think of the asterisk as a mini explosion, representing instability.) The kind of AO's from which the MO's came is indicated by a subscript. Then we can write the two molecular orbitals as

             
                    I                                        II

(These are expressions for the wave functions, and the electron densities in the two MO's are found by squaring them.)