By Harold McGee
A second kind of chemical bond, called covalent (from the Latin, “of equal power”), produces stable molecules. When two atoms have roughly similar affinities for electrons, they will share them rather than gain or lose them entirely. In order for sharing to occur, the electron clouds of two atoms must overlap, and this condition results in a fixed arrangement in space between two particular atoms, which thus form a stable combined structure. The bonding geometry determines the overall shape of the molecule, and molecular shape in turn defines the ways in which one molecule can react with others.