Covalent bonds

The triple bond in N₂ is 110 pm long; the single bond in ethane is 154 pm. More shared pairs pull atoms closer.

When two nonmetal atoms meet, neither is willing to give up electrons entirely. Both pull hard, so instead of a transfer they share. Each atom puts one electron into a pair that sits between the two nuclei, and both nuclei are attracted to that pair. The shared pair is the covalent bond. It is how nearly every substance in a living thing is held together, from water to DNA.

Counting shared pairs

Take methane, CH₄. Carbon has 4 outer electrons and needs 4 more. Each hydrogen has 1 and needs 1 more. Carbon shares one pair with each of four hydrogens. Now carbon counts 8 electrons around it and every hydrogen counts 2. Everyone has a full shell, and the molecule has four single bonds. The number of bonds an atom usually forms is called its valence: hydrogen 1, oxygen 2, nitrogen 3, carbon 4. Those four numbers let you build most small molecules by hand.

For any small molecule you can check a structure by counting electrons. Add up the outer electrons of every atom: water has 1 + 1 + 6 = 8. Each bond uses 2 of them, and whatever is left over sits in lone pairs. Water has two O-H bonds, using 4 electrons, which leaves 4 electrons as two lone pairs on oxygen. Oxygen then counts 8 around it and each hydrogen counts 2. Ammonia works the same way: 5 + 3 = 8 electrons, three N-H bonds and one lone pair. These lone pairs turn out to decide the shapes of the molecules in the next unit.

Double and triple bonds

Sometimes one shared pair is not enough. In an oxygen molecule, O₂, each atom has 6 outer electrons and needs 2 more, so the two atoms share two pairs: a double bond, O=O. In nitrogen, N₂, each atom needs 3, and they share three pairs: a triple bond, N≡N. The model draws each shared pair as a separate stick, so you can count them. Carbon can do all three. Ethane, C₂H₆, has a C-C single bond, ethene, C₂H₄, a C=C double bond, and ethyne (acetylene), C₂H₂, a C≡C triple bond.

More shared pairs hold the atoms closer and more tightly. The measured carbon-carbon distances from the NIST database show it clearly: 153.6 pm in ethane, 133.9 pm in ethene and 120.3 pm in ethyne. The nitrogen triple bond, 109.8 pm, is so strong that nitrogen gas hardly reacts, even though it makes up most of the air we breathe. Plants cannot use it directly; they rely on bacteria and on lightning to break it.

Molecules, not lattices

Covalent bonds join a fixed number of atoms into a molecule with a definite shape. The bonds inside a water molecule are strong, but the forces between one water molecule and the next are much weaker. That is why many covalent substances are gases or liquids at room temperature and melt or boil at low temperatures: you only need to separate the molecules, not break them. Methane boils at about −161.5 °C. A few covalent substances, such as diamond and quartz, are giant networks of bonds with no separate molecules, and those are among the hardest materials known.