If known or measurable, the radius of each element is quoted (in pm, 10éåç metres).
The covalent radius is quoted for single covalent bonds (after Pauling). The covalent radius of an atom is that atoms contribution to the length of a covalent bond, and is broadly an indication of the radius of the inner core of an atom, and may not have absolute significance. Their values are chosen so that reasonably reliable bond lengths can be predicted in a number of molecules. A covalent bond is a bond between atoms in which each provides one electron to a shared pair. In covalent bonding, the atoms remain electrically neutral and do not acquire charges as in ionic bonding. The covalent radius is smaller for multiple and polar bonds, and any ionic character will also shorten the radius. It will be noticed, that broadly speaking, taking the elements in their group numbers, covalent radius increases with atomic weight, and that the alkali metals have relatively large radii, with caesium having the largest, whilst the transition metals have smaller, but consistent radii. Hydrogen has the smallest radius, followed closely by boron, carbon, nitrogen, oxygen and fluorine. The covalent radius is consistently slightly smaller than the atomic radius.
Unlike the ionic bond, where the attractive force decreases as the inverse square of separation distance between atoms, the covalent bond falls off quicker as the inverse cube of distance in molecules like Hö and other saturated bonds. Also, unlike the ionic bond, which is spherically symmetric, the covalent bond can be directional and depends upon the outer electronic configurations of both participating atoms. The strength of the covalent bond is typically 100kcal/mole.
Generally, the lowest energy arrangement of atoms with LIKE or zero charges is that of either a hexagonal close-packed or face centred cubic lattice where each has 12 neighbours, whereas the lowest energy arrangement for two differing atoms with OPPOSITE charges is that where the unlike charges are arranged alternately in a cubic lattice, where each has but 6 neighbours. Many ionic salts including sodium chloride, NaÇClé, have such a structure.
