Covalent Radius
Definition and meaning of Covalent Radius in chemistry.
Covalent radius is half the distance between the nuclei of two identical atoms joined by a single covalent bond, used as a measure of atomic size.
In more detail
Because electron clouds do not have a sharp boundary, covalent radius is defined operationally from measured bond lengths rather than from a fixed atomic edge. Covalent radii decrease across a period as increasing effective nuclear charge pulls bonding electrons closer to the nucleus, and they increase down a group as additional electron shells are added. The property is roughly additive: the bond length between two different atoms can be estimated by summing their individual covalent radii, which makes covalent radii useful for predicting bond lengths in molecules that have not yet been measured directly.
Key facts
| Field | General Chemistry |
|---|---|
| Typical unit | picometers (pm) |
| Trend across a period | decreases (left to right) |
| Trend down a group | increases |
In the Cl2 molecule the Cl-Cl bond length is 198 pm, so the covalent radius of chlorine is taken as half that value, 99 pm; this value can then be added to carbon's covalent radius (77 pm) to estimate the C-Cl bond length in chloromethane as about 176 pm.
Frequently asked questions
How is covalent radius different from ionic radius?
Covalent radius describes a neutral atom bonded to an identical neutral atom, while ionic radius describes a charged ion in a crystal lattice; forming a cation shrinks the radius and forming an anion expands it relative to the neutral atom.
Can covalent radii be used for bonds between different elements?
Yes, approximately: the bond length between two different atoms is estimated as the sum of their individual covalent radii, though this additivity breaks down somewhat for bonds with significant polarity or multiple-bond character.