Slater's covalent radii agree nicely with experiment when the effective principal quantum number and the screening constant for the outermost electron are taken from fits of the hydrogen-like energy level expression to ionization energies in isoelectronic series. This establishes a simple connection between the two prominent periodic properties of atoms, namely, their radius and (effective) ionization energy. (C) 2014 Elsevier B.V. All rights reserved.