covalent bond

Two atoms can hold hands in chemistry by sharing electrons, and that shared grip is a covalent bond. Think of two children each contributing one hand to a shared clasp: neither lets go, both are held, and the pair stays together. In a covalent bond two atoms each put an electron into a shared pair that sits between their nuclei, and the mutual attraction of both nuclei to that pair glues the atoms together. This is the bond that holds nearly every organic molecule together.

More precisely, a covalent bond forms when two atoms share one or more pairs of electrons so that each gets closer to a filled outer shell. One shared pair is a single bond, two shared pairs a double bond, three a triple bond. The shared electrons are not stationary; they spread into a region of high probability between (and around) the two nuclei. Covalent bonds contrast with ionic bonds, where one atom hands an electron entirely to another and the two then attract as opposite charges. When the atoms sharing are different, the sharing is lopsided — a polar covalent bond — and at the extreme of inequality it shades into an ionic bond.

Covalent bonding matters because it is the bond of organic chemistry: carbon shares rather than transfers electrons, and almost every C-C, C-H, C-O and C-N link in every organic molecule is covalent. Understanding that a bond is a shared electron pair is the key to curved-arrow mechanisms, where reactions are simply the making and breaking of these pairs. A common misconception is to picture the shared electrons as fixed dots sitting still on a line between the atoms; they are really a smeared-out cloud, densest between the nuclei.