The earlier models of covalent bonds taught us that covalent bonds are formed when (1) the shared electrons used to formed the covalent bond must each come from the two constituent atoms and (2) the non-metals seek to attain an octet, hence search to have 4 electron pairs aroud it, therefore resulting in a maximum of 4 single bonds. These knowledge is still applicable, however, there are extensions.
(A) Dative covalent bonds.
Dative covalent bond are formed when an electron pair used to form a covalent bond between two particles come from just one source. Hence, this implies that one of the particles needs to have a pair of non-bonding electrons for donation and the other particle contains an empty orbital to accept the electron pair. I have illustrated using an example of an adduct formed by AlCl3 and NH3.
My second illustration of dative bond formed is using just AlCl3. This compound usually exists as dimer and it is because Cl has available lone pair while Al has an empty orbital to accept the electrons. Interestingly, the Cl does not donate its lone pair of electrons to the Al it is attached to in a molecule; it rather donates its lone pair to a Al which is found on another molecule. This is because to form a double bond between Al and Cl is unfavourable as compared to having to form a dative covalent bond.
You may learn more about why AlCl3 is a covalent compound over here.
(B) Expansion of octet - Having more than 8 electrons.
Elements found in Period 3 and below show an unique property. These elements show that the expansion of the octet is possible. For example, sulfur can exist as SO2. In this molecule, S satisfies octet rule. While sulfur can also exist as SO3 and in this molecule, S has more than 8 electron around it. This shows that S has expanded its octet (This is illustrated below.) It is because S contains
empty low-lying 3d-orbtials which are available to accept more electrons.
Lastly, one needs to be mindful, when the octet is expanded, we will still get even number of electrons - all the electrons are paired. Hence, we will not observe 9 electrons or 11 electrons.
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Article written by Kwok YL 2009.
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