In the process of studying covalent compounds, most students, myself included, have undoubtedly wondered, "Why do elements share electrons? Why wouldn’t atoms just gain electrons from another atom, as happens when ionic compounds are formed? Why should it be any different?"
Well, the reason that non-metals have to share electrons with each other is directly related to electronegativity. To put it simply, electronegativity is a measure of how much an element pulls electrons away from other elements it is bonded to. Generally, metals have very low electronegativities (meaning they don’t really try to gain electrons) while non-metals have high electronegativities (meaning they have a strong need/urge to gain electrons). The reason for this trend is the octet rule, which states that all elements strive to achieve the same number of electrons as the nearest noble gas, due to noble gases being far more stable. When metals bond with non-metals, the result is fairly obvious to see. Ionic compounds will be formed because the metal atoms have an excess of electrons and will easily lose them to non-metals that require electrons.
It’s altogether a whole new ball game, however, when two non-metals bond with each other. Instead of simply having one element give electrons to another, we end up with a situation whereby we have two elements with about the same electronegativity. As a result, neither element is able to gain electrons from the other. As a result, if either of them aims to attain noble gas structure, they end up having to share electrons rather than transfer them.
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