A covalent chemical bond is formed when atoms share a pair of electrons. For example, hydrogen has one electron and carbon has four electrons in the grouping known as methane, the main constituent of natural gas. As shown below, four covalent bonds can be formed between the carbon and hydrogen atoms.

When carbon has four bonds, it is called tetravalent. A simpler, compressed representation is CH₄. As shown below, dashes rather than dots can be used to represent pairs of electrons.

When carbon has three bonds instead of four, it is called trivalent. This grouping is known as the methyl radical. As shown below, if a carbon atom has just three bonds to hydrogen atoms, one electron would remain unshared and and unpaired.

The phenyl group is very important to organic chemistry. As shown below, it consists of six carbon atoms bonded to five hydrogen atoms, with double bonds (four electrons) between some of the carbon atoms.

Moses Gomberg discovered triphenylmethyl, the first organic free radical with a measureable lifetime. As shown below, three phenyl groups replace the three hydrogen atoms in the methyl group triphenylmethyl.