Before you start

Organic chemistry is the chemistry of carbon compounds: fuels, plastics, medicines, food and you. This lesson recaps the bits of general chemistry it leans on, so you can start here. The General chemistry course covers them properly.

Atoms are made of protons and neutrons in a nucleus, with electrons around it. The outer electrons, the valence electrons, do the bonding. Atoms bond until they have a full outer shell, which usually means eight electrons (two for hydrogen).

In a covalent bond two atoms share a pair of electrons. Each element makes a typical number of bonds: hydrogen 1, oxygen 2, nitrogen 3, carbon 4, and the halogens (fluorine, chlorine, bromine, iodine) 1. A double bond shares two pairs and a triple bond three.

Electronegativity is how hard an atom pulls on shared electrons; oxygen, nitrogen and the halogens pull harder than carbon and hydrogen. When the pull is uneven the bond is polar, with a slightly negative end (δ−) and a slightly positive end (δ+). Carbon and hydrogen pull about equally, so C–H bonds are close to nonpolar.

A mole is 6.022 × 10²³ molecules, and the molar mass is the mass of one mole in grams: add up the atoms using H = 1, C = 12, N = 14, O = 16. A chemical formula like C₂H₆O lists how many of each atom a molecule has.