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In lecture, we learned that formal charge indicates the gain or loss of an electron while forming a covalent bond. However, I thought that covalent bonds did not gain or lose electrons, but rather shared electrons between elements. Could someone clarify what exactly is meant by formal charge?
This could be incorrect, but I think this has to do with the idea that all bonds have some ionic and some covalent characteristics. A pure covalent bond has a formal charge of 0, and that's why when trying to draw Lewis structures we look for the combinations that have the closest formal charge to 0. These are the lowest energy configurations and thus will be the combinations that are experimentally observed. The 'gain or loss' of electrons represents an unequal sharing of charge (electrons) that atoms in covalent bonds naturally seek to avoid. However, some 'covalent' compounds will have a little unequal sharing of charge (and thus a formal charge other than 0), and that is where their ionic characteristics come from.
From what I understood, since covalent bonds share electrons the formal charge formula indicates s/2 to represent an equal sharing. However, in reality there are atoms with higher electronegativities in many cases which cause unequal sharing that is a thing the formula does not account for.
Keep in mind the difference between Formal Charge versus oxidation states. What formal charge attempts to do is determine the lowest possible energy configuration for a covalent compound. That is, the most stable. If you're thinking about oxidation states, it is an exaggeration of a charge in a molecule to show around which nuclei electrons tend to spend most time around.
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