Expansion of Octet. Why do elements do it?

Most of the elements do not have the noble gases' electronic configuration. Since the latter is generally unreactive, we have come to the conclusion that the octet configuration is stable and that the other elements would aim to achieve it. These elements can only achieve the octet configuration via chemical bonding; either forming of a covalent bond or an ionic bond with another atom.

However, we also noticed that there are compounds which clearly show that the element can form more bonds such that they have more than the 8 electrons around it. Compounds such as PCl₅ and H₂SO₄; where phosphorous and sulfur have 10 and 12 electrons around them respectively. Glaringly, all the elements which are found in the first and second period of the periodic table does not form compounds in which they have more than 8 electrons around them.

Therefore, these observations together with the knowledge of quantum shells, subshells and orbitals, we are able to come to the conclusion that elements found from Period 3 and beyond, have available empty orbital which must be energetically accessible to accept electrons for bonding. Thus, these elements are able to accept more electrons to bond with other elements. This results in the expansion of octet. In the case of PCl₅, phosphorous must have caused one of its valence electron to be excited to the 3d subshell (its energetically accessible orbital), so that it has 5 orbitals available for bonding and hence able to form a single bond each with 5 chlorine atoms.

However, is it that random, is it simply that elements which have energetically accessible orbital will exercise that privilege to use them? How do we account for compounds such as XeF₂?  Xe a clear noble gas element and yet it is willing to expand its octet, why would it do so? The expansion of the octet results in the elements to be able to make use of their energetically accessible empty orbitals. These orbitals are energetically slightly higher than the orbitals which the valence electrons are found. Hence, if the element wants to expand its octet, it will excite some of its valence electrons to the empty and energetically accessible orbital, so that they can form a bond with another atom via equal sharing of electrons or accept a pair of electrons from the donor to form a dative bond. If this is a favourable process, the energy from the formation of the bond must have compensated the energy needed to move a valence electron out of it the orbital that it resides in.

This is the complete reason to why the Period 1 and Period 2 elements do not expand their octet.  Their next empty orbital belongs to a different principal quantum shell. Hence, to make use of it for bonding would require a large amount of energy, and it is not compensated by the energy formed from the bond formation. Therefore, the combination of these two reasons allow us to observe expansion of octet in some elements and not all of them.

Interestingly, the cation of an ionic compound generally do not favour expansion of octet which results in formation of another neutral compound.  (For example adding more F₂ to MgF₂ to form MgF₄.) This is because it results in Mg to use more than 2 valence electrons for bonding.  This is energetically meaningless for the atom as it will have to invest in large amount of energy to make use of an inner shell electrons for bonding.  Hence, if cation wants to expand its octet, then it needs to have energetically accessible empty orbital to accept electron pairs from a donor.

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^{Article written by Kwok YL 2011.}

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