Bonding orbitals in MO theory always concentrate an excess of electron density between the atoms being bonded, and in this regard the MO theory is no different from the old qualitative electron-pair ideas of a bond. But only MO theory recognizes that electron pairs can tear a molecule apart as well, if they are placed in antibonding orbitals. The essential factor in determining bonding is the net excess of bonding electrons over antibonding electrons. If these bonding and antibonding electrons counterbalance one another, then the molecule will not form, as we saw for He, Be, and Ne. Nothing in MO theory contradicts the simpler electron-pair theory, or we would have grave doubts about its validity. But even more, MO theory explains molecular properties about which the simpler theory has nothing to say-for example, the magnetic behavior of the O molecule with its two unpaired electrons, and the planarity of molecules with double bonds. This chapter has been a pictorial and qualitative introduction to molecular orbital theory, but the theory also has a mathematical and quantitative side that permits the calculation of energy levels and of ionization energies, spectra, and reactivities of molecules.