Bond length, also known as bond distance, is the distance between the nuclei of two covalently bonded atoms within a molecule, usually expressed as picometer (pm) and also Angstrom units. The farther the nuclei from each other, the weaker will be the bond. This naturally means the bond length would increase with the atom’s size. The length could be as short as 74 pm and go up to 200 pm for big atoms. The usual range is 100-200 pm.

Measuring Bond Length

Atoms aren’t stationary within a bond. They could move around or bounce when in a bond, which depends on the total number of bonds involved. The intensity of atom movement wouldn’t be the same in a single and double bond. But, at a certain point, the atoms would come to a static position, which is the state where the highest amount of energy exists between the two atoms. The atoms are not too close or too far from each other – their positioning is just right at this point. This is precisely the position where bond length is measured. Also, the energy holding the bond together is measured.

Factors Determining Bond Length

Factors ascertaining bond length are the bonded atoms’ sizes, electronegativity differences between them, and the bonded molecules’ overall structures. Electronegativity indicates the level of attraction existing between two bonded atoms. Generally, a higher bond order means a shorter bond length or a stronger bond. Bond order is basically the total number of electrons involved in a bond. For example, the bond order for a double bond would be four since there are two pairs of electrons involved in the bond.

Bond length of two similar atoms divided by two is referred to as covalent radius. The sum of a couple of covalent radii is typically the single bond length. However, just adding covalent radii isn’t everything there is to bond length. Besides the aforementioned factors, there are quite a few other pertaining aspects as well such as resonance, steric effect and hybridization.

Resonance denotes delocalized electrons that make it difficult to express a molecule’s structure as one single structure. Several hypothetical structures are used to express the resonating or moving electrons. Steric effect is essentially the repulsion effect created between two atoms in a molecule when they come too close to each other or when the atoms infringe upon each other’s personal space. Hybridization is the combining of multiple atomic orbitals or the space surrounding the nucleus of an atom.