How do methyl radicals combine to form ethane?

Methyl radicals ($·{\text{CH}}_3$) are highly reactive species that contain a single, unpaired electron. Because of this unpaired electron, they are inherently unstable and possess a fleeting independent existence.12

To achieve stability, methyl radicals undergo a process known as dimerization. In this reaction, two methyl radicals combine by sharing their respective unpaired electrons to form a single covalent bond, resulting in the stable molecule ethane (${\text{H}}_3{\text{C—CH}}_3$). This process is a spontaneous way for these radicals to pair all of their electrons and reach a lower energy state.21