Covalent bonds

Between two non-metals, neither atom wants to give electrons away — so they share. A shared pair of electrons is a covalent bond. Share one pair and you have a single bond; two pairs make a double bond; three make a triple bond, which is stronger and shorter still.

Water is two hydrogen atoms each sharing a pair with one oxygen. Carbon dioxide is a carbon sharing two pairs with each of two oxygens (double bonds). The number of bonds an atom forms follows its valence: H makes 1, O makes 2, N makes 3, C makes 4.