In chemistry, a multiple bond is made when two atoms share more than one pair of electrons. The number of pairs in a multiple bond is called its bond order. The double bond is the most common type of multiple bond.
In modern molecular orbital theory, the bond order is defined as half of the difference between the number of electrons in bonding orbitals and the number of pairs in antibonding orbitals. Bond order must be a positive number to make a stable molecule.
In more complicated molecules with delocalised electrons, bond order does not need to be a whole number. For example, a bond between carbon atoms in benzene has one sigma bond with two electrons, and the delocalised pi bonds that share six electrons among all six carbon-carbon bonds in the ring. The bond order of two carbon atoms in benzene is 1.5, with one whole sigma bond and the pi bond acting like half of a bond.
Higher bond order holds two atoms together more strongly. Multiple bonds have higher bond dissociation energy, making them harder to pull apart. The distance between atoms also gets shorter as bond order increases.
Multiple bonds between specific atoms are more stable with larger bond order, but this does not mean that molecules are less reactive. For example, the bonding energy of the C=C bond in ethylene is 729 kilojoules per mole, which is much stronger than the C−C bond in ethane (377 kJ/mol). However, the hydrogenation of ethylene also creates two C−H bonds (421 kJ/mol), so the total energy of the product is higher even if the bond between carbon is weaker. Thus the reaction is exothermic.