Bond parameters

Covalent bonds are described by four bond parameters:

• Bond length: the equilibrium distance between the nuclei of two bonded atoms (H–H in H₂ is 74 pm; Cl–Cl in Cl₂ is 199 pm). The covalent radius is half the bond length between two identical atoms.
• Bond angle: the angle between the bonds around a central atom, showing the shape of the molecule (H–O–H in water is 104.5°).
• Bond enthalpy: the energy needed to break one mole of bonds of a kind, between atoms in the gas phase. H–H needs 435.8, O=O 498 and N≡N 946 kJ mol⁻¹.
• Bond order: the number of bonds between two atoms. H₂ has 1, O₂ has 2, N₂ has 3.

In a molecule with several identical bonds, the bonds may not all need the same energy to break. In water, breaking the first O–H bond needs 502 kJ mol⁻¹, but the second only 427 kJ mol⁻¹, because the atoms around it have changed. So for such molecules we use the average bond enthalpy: (502 + 427) ÷ 2 = 464.5 kJ mol⁻¹ for O–H in water.

Isoelectronic molecules and ions (with the same number of electrons) have the same bond order.

The general rule linking these: the higher the bond order, the stronger the bond (higher bond enthalpy) and the shorter it is. A triple bond holds two atoms more tightly and closer together than a double bond, and a double bond more than a single bond.