half the distance between the nuclei of identical neighbouring atoms. An atom has no rigid spherical boundary, but it may be thought of as a tiny, dense positive nucleus surrounded by a diffuse negative cloud of electrons. The value of atomic radii depends on the type of chemical bond in which the atoms are involved (metallic, ionic, or covalent bond). When the neighbouring atoms are not alike, as in sodium chloride (salt), part of the observed distance between atoms is assigned to one kind of atom and the rest to the other kind.

The metallic radius of sodium atoms bonded together in a chunk of sodium metal is larger than the ionic radius of sodium in the compound sodium chloride. In sodium chloride, each sodium atom has lost an electron to become a sodium ion (charged atom) of unit positive charge. On the other hand, each chlorine atom has gained one electron to become a chloride ion of unit negative charge. The ionic radius of chlorine is nearly twice as great as the radius of a neutral chlorine atom. The bond between the pair of chlorine atoms in a chlorine molecule and between the carbon atoms in diamond are examples of covalent bonds. In these and similar cases, the atomic radius is designated as a covalent radius.

The distances between atoms and ions have been determined very accurately, for example, by X-ray diffraction analysis of crystals. Typical atomic radii have values of about one or two angstrom units. (One angstrom, 1 Å, equals 10⁻¹⁰ metre.)