The size of an atom has a great effect on a variety of properties of the element and its compounds. Several different radii can be measured. The atomic radius is defined as half the distance betrween the nuclei of two adjacent atoms of the same element. This is most accurately determined for solids.

Other radii that may be reported include covalent radii, ionic radii, and van der Waals radii of elements.

The covalent radius is half the distance between the nucleus of the reported element and the nucleus of another atoms when in a covalent bond. The reported value may vary based on the other element involved in the bond.

The van der Waals radius is half the distance between nuclei of the closest non-bonding approach of two atoms of the same element.

Ionic radii are measured for ions of a specific charge in ionic solids. We will look at this briefly in connection with atomic radius [local].

A 3-D graph of atomic radius variation of periods 1-6. Note the general trend in the periods and in the groups.

Another graph shows a close look at the variation in atomic radius in periods 4,5, and 6. Notice the change as the d subhells and the 4f subshell fill. Note where exceptions to a smooth change. Be prepared to connect those exeptions to the electron configuration of the elements.

Very closely linked to the atomic radius is the radius of ions formed from the atom. In a qualitative sense, if an electron is removed there are fewer electrons for the same nuclear charge to attract. As a result when a neutral atom loses an electron the remaining electrons are more strongly attracted and are pulled closer to the nucleus. This effect is even greater when all the electrons in the valence level are removed. Now the number of energy levels has been decreased by one and the highest energy (valence) level is one level lower and thus has a smaller radius of maximum probability. As a result the radii of positive ions are smaller than the corresponding atom. The more electrons that are removed in forming ions the smaller the radius becomes.

The reverse occurs when a negative ion is formed by gaining an electron. The electron added is in the same energy level as the other valence electrons but the effective nuclear charge is lower in comparison to the number of valence electrons. The attraction of the nucleus for each electron is less than before and the negative ion has a larger radius than the neutral atom. Forming -2 and -3 ions increases the size of the ion even more. Comparison of atomic and ionic radii. Selected ionic radii.