How is a single covalent bond formed?

Just like in ##H_2##, ##Cl_2##, ##CH_4##.

A single covalent bond is formed when 2 atoms share only 1 pair of electrons between them, thus attaining more stability. When a strong sigma bond is formed between the two atoms, as a form of a molecule or compound, the atomic orbitals directly overlap between the nuclei of two atoms.