A simple rule of thumb from inorganic says that when comparing 2 atoms, the atom that is more electronegative will have atomic orbitals lower in energy when compared to another atom of lower electronegativity. Cl- is a lot more electronegative than Na so its orbitals are much lower than Na's atomic orbitals. Sodium chloride is also an ionic bond. If you look at it from a molecular orbital theory viewpoint, if the energies of orbitals are so different than the only bonding interaction that can occur is ionic bonding. In order for covalent bonding to occur you must satisfy 3 rules, proper distance, similar symmetry, and similar energies. While the 3p of the Cl and the 3s of the Na atom have similar symmetries, they do not satisfy the rule for similar energies. Thus no covalent bond can form, only an ionic bond.
Hello Gravenewworld, thank you very much for your clear explanation.. I understand why the energy of 3s orbital of Na is higher than that os 3s orbital of Cl...though I would like to ask: as 3p is different from 3s...why we can compare them this way..or is it that for elements with higher electronegativity, all their orbitals in the same shell is lower in energy than those atoms with higher energies?
my questoin is "Why the energy of 3s electron of Na is much higher than that of the 3p electrons of Cl?", and you told me that it is because Na has lower electronegativity than Cl...but I suppose 3p is not 3s, so we can't compare them this way, isn't it?
You can compare 3p orbitals with 3s orbitals because they are similar in symmetry. The question is will the 3pz orbital of the Cl atom and the 3s orbital of the Na atom interact to form a covalent bond? Whether they will form a covalent bond once again comes down to the 3 rules I listed earlier. We already said that they are similar in symmetry, and proper distance is usually never a problem. The reason that the 3pz and the 3s orbital do not interact to form a covalent bond is still because of the differences in energy between the two orbitals. The electronegativity difference is so great that it causes the energy differences between the 3pz and 3s orbitals to still be large, so without similar energies a covalent bond can not form, which is why NaCl forms from ionic bonding.