9. Molecular Orbitals and      Molecular Structure   Previous PageNext Page
     Larger Diatomic Molecules

The first principle is reasonable. Atoms cannot combine when they are far from one another. However, there is a more practical aspect. A 2s orbital is larger than a 1s, and when two atoms are brought close enough for overlap between 2s orbitals, the smaller 1s orbitals are still too far away to overlap appreciably. We can leave them as separated atomic orbitals, each with a pair of electrons around each atomic nucleus. This is another way of saying that when the outer electron shells of an atom are involved in bonding, the filled under shells take no part and can be ignored. For an atom in any row of the periodic table, the electrons of the noble gas that brought the preceding row to a close can be regarded as an unchanging atomic core, and neglected in bonding. This makes life much simpler.
The principle of similar energies would have ruled out combinations between 1s orbitals and 2s or 2p orbitals from the same kind of atoms. It can even take us one step further, and tell us that in the simplest treatment we do not have to worry about combining 2s and 2p orbitals on different atoms of the same type; the energy difference between s and p orbitals is enough to make their combination unlikely.

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