Exceptions to the Octet Rule - QS Study
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Sometimes covalently-bonded atoms have a share in fewer than 8 electrons e.g. Be in BeCl2, B in BH3. Be and B have very high ionisation energies, and so do not form ionic compounds easily. But, they do not have enough electrons to form sufficient covalent bonds to make an octet.

a1

Semi-stable compounds can be formed if all the available electrons are shared. Atoms with less than a complete octet can readily accept electron pairs to ‘complete their octets’. So, they react easily with other substances which have an electron pair available for forming co-ordinate bonds.

E.g. BeCl2 + 2Cl → BeCl42-

a2

Sometimes covalently-bonded atoms have a share in more than 8 electrons.

E.g. P in PCl5 and S in SF6

How can the octet rule be violated? The octet rule arises because the s and p orbitals can take up to 8 electrons. However, once we reach the third row of elements in the periodic table we also have d-orbitals, and these help take the extra electrons.