In a covalent bond, the electron clouds surrounding the atomic nuclei overlap. Formation of a covalent bond between two Hydrogen atoims. Image from W.H. Freeman and Sinauer Associates, used by permission. By sharing the electrons with other atoms, Carbon can become a happy atom,. alternately filling and emptying its outer shell. Formation of covalent bonds in methane.
Carbon needs to share four electrons, in effect it has four slots.
Each hydrogen provides an electron to each of these slots. At the same time each hydrogen needs to fill one slot, which is done by sharing an electron with the carbon. Image from W.H. Freeman and Sinauer Associates, used by permission. Carbon contributes an electron, and Hydrogen contributes an electron. The sharing of a single electron pair is termed a single bond.