Carbon needs to form four covalent bonds to complete its outer shell. This is because carbon has four valence electrons, which are the electrons in the outermost shell of an atom that participate in chemical bonding. When carbon atoms bond with other atoms, they share their valence electrons in order to achieve a stable electron configuration. Carbon atoms can share their valence electrons in a variety of ways, but the most common way is to form covalent bonds. In a covalent bond, two atoms share one or more pairs of electrons. When carbon forms a covalent bond, it shares one of its valence electrons with another atom, and that atom shares one of its valence electrons with carbon. This creates a strong bond between the two atoms, and it allows carbon to achieve a stable electron configuration.