Bond order refers to the number of chemical bonds between two atoms, and relates to the stability of the bond. Bonds are classified as single, double or triple. For example, diatomic nitrogen (N2) has a triple bond between the two atoms (N≡N) while acetylene (C2H2) has a bond order of three between the two carbon atoms and single bonds between the carbon atoms and the hydrogen atoms (H−C≡C−H).
Bond length is inversely proportional to bond order. This makes intuitive sense; a triple bond is stronger than a double bond, so the atoms in such an arrangement are closer together than two atoms joined by a double bond, which in turn are separated by a smaller distance than the atoms in a single bond.
Bond Order for Whole Molecules
Bond order in analytical chemistry normally refers to the bond order of the whole molecule, not just to individual bonds.
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A simple formula is used to calculate this quantity: Add together the total number of bonds, counting 1 for a single bond, 2 for a double bond and 3 for a triple bond, and divide by the total number of bond groups between atoms_._ Often, this yields a whole number, but not always. Bond order may be regarded as a rough measure of the average strength of a molecule's bonds.
Examples of Bond Order Calculations
Molecular hydrogen (H2) has the structure H−H. There is one single bond and a total of one bond group, so the bond order is simply 1.
Acetylene (C2H2), as noted, has the molecular structure H−C≡C−H. The total number of bonds is 1 + 3 + 1 = 5, and the total number of bond groups is 3 (two single bonds and a triple bond). The bond order for acetylene is therefore 5 ÷ 3, or 1.67.
A nitrate ion (NO3-) has one double nitrogen-oxygen bond and two single nitrogen-oxygen bonds for a total of 4 bonds distributed across three bond groups. The bond order of nitrate is therefore 4 ÷ 3, or 1.33.