The radius of an atom is described as the distance from its nucleus to its outermost electrons. Although it is impossible to know the exact position of these electrons, a very close approximation of the radius of an atom can still be determined by measuring the distance from its nucleus to that of another atom it is bonded with. In a covalent bond -- formed by shared electrons -- the two atoms are assumed to be the same size, and the distance between the nuclei of the two atoms can be divided in half to find their radius. In the case of ionic bonds, one atom is larger than the other, and the radius of one of the atoms must be known in order to determine the radius of the other.
Determine what type of bond exists between the two atoms; the radius will be calculated differently depending on whether or it is covalent or ionic.
Divide the distance between the nuclei of the atoms by two if the bond is covalent. For example, if you know the distance between the nuclei of two covalently bonded atoms is 100 picometers (pm), the radius of each individual atom is 50 pm.
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Subtract the radius of one of the atoms from the total distance between the nuclei if the bond is ionic. For example, if the radius of one of the atoms is 60 pm, and the distance between the nuclei of the two atoms is 160 pm, the radius of the other atom is 100 pm.