Atomic Radii and Effective Nuclear Charge

An atom’s size is dictated by the electrons or their orbitals. However, orbitals do not describe a confined space, but rather the statistical probability of where an electron can be found. So how is the atomic size defined, and what influences it?

An atomic radius can be described in two ways. Nonbonding atomic radius, or van der Waals radius of an atom, is one-half of the distance between adjacent nuclei in the atomic solid. 

Conversely, a bonding atomic radius, or covalent radius, distinguishes between metals and nonmetals. In metals, the radius is described for atoms in their crystal structure as one-half of the distance between the centers of two neighboring atoms. 

In nonmetal, diatomic molecules, the radius is described as one-half of the distance between the centers of bonded atoms.

The periodic table depicts variations in covalent radii that are often called atomic radii, which are influenced by two factors; the number of principal energy levels of valence electrons, and the effective nuclear charge. 

The trend in atomic radii for main group elements down the columns is depicted here.

Moving down a group, the principal quantum number, n, increases by one for each element. T