The bond enthalpy of hydrogen is 435.8 kJ mol⁻¹.

A covalent bond forms when two half-filled orbitals overlap. Head-on overlap along the line of the nuclei gives a sigma bond. Sideways overlap of parallel p orbitals gives a pi bond, which is weaker. A single bond is one sigma bond, a double bond one sigma and one pi, and a triple bond one sigma and two pi. The bond enthalpy is the energy needed to break one mole of bonds into atoms.