Tabla de Contenidos
The concept of covalent radius is related to the size of an atom when it is part of a covalent bond with another atom. As seen in the figure, the covalent radius is defined as half the distance between the nuclei of the two atoms that share a covalent bond. Covalent radius is expressed in units of picometers (pm) or angstroms (Å) (1 Å = 100 pm). Because the covalent radius of an atom is defined in terms of its association with other atoms, its value is not unique and will depend on its bonding with other atoms; As we will see later, the covalent radius of an element is different if it is combined with a single covalent bond or with a double or triple covalent bond.
Atomic radius, ionic radius, metallic radius and covalent radius
The concept of atomic radius of an element is associated with the distance between its nucleus and the most distant electrons. The shape of the atom can change when bonding with other atoms, so the atomic radius is not a consistently defined physical parameter, and therefore different radii are defined depending on the chemical bonding of the atom. In general, the atomic radius of a chemical element can be defined as half the distance between its nucleus and that of another atom with which it is chemically bonded.. As we have already seen, the covalent radius is defined the same as the atomic radius when the atom is linked by a covalent bond, and the ionic radius will be the atomic radius when the atom establishes an ionic bond, as in a crystal lattice of a salt. In the same way, the metallic radius is definedas the atomic radius when the atoms are linked by a metallic bond. For example, in the case of sodium (Na), the atomic radius is 180 pm, the covalent radius is 154 pm, and the metallic radius is 186 pm. In the case of calcium (Ca), the atomic radius is also 180 pm, the covalent radius with a single bond is 174 pm, the covalent radius with a triple bond is 133 pm, and the metallic radius is 197 pm. Another example is that of iron (Fe), whose atomic radius is 140 pm, the covalent radius with a single bond is 125 pm, the covalent radius with a triple bond is 102 pm, and the metallic radius is 126 pm.
Sources
- Pyykko, P., Atsumi, M. Molecular Single-Bond Covalent Radii for Elements 1-118 . Chemistry: A European Journal 15: 186–197, 2009. doi: 10.1002/chem.200800987
- Sanderson, R.T. Electronegativity and Bond Energy . Journal of the American Chemical Society 105 (8): 2259–2261, 1993. doi: 10.1021/ja00346a026