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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