2.3.3: Covalent and Ionic Radii

• In nonpolar atomic radii, the radius of an atom is derived from the bond lengths within nonpolar molecules; one-half the distance between the nuclei of two atoms within a covalent bond.
• In van der Waals radius, the radius of an atom is determined by collision with other atoms.
• In crystal radii, the atomic or ionic radius is determined using electron density maps from x-ray data.
• The size of an atom or ion is a "fuzzy" measure, and the radius under a different set of conditions will change slightly.
• Atomic size generally decreases gradually from left to right across a period.
• As nuclear charge (Z) increases, we expect the effective nuclear charge (Z*) of the valence electrons to also increase, pulling electrons closer to the nucleus. However, with each additional unit of Z, there is an additional electron, causing a gradual decrease in atomic radius across a period.
• Atomic size generally increases going down a group.
• Ionic radius varies with the charge of the ion and the electron configuration.
• Cations have the same Z but fewer electrons, resulting in a significant increase in effective nuclear charge and all other electrons are now more strongly attracted to the nucleus with a lower energy level. This results in a contraction of size from the atom to cation.
• Anions have the same Z but more electrons, resulting in a decrease in effective nuclear charge corresponding to a decrease in attractive force. This results in an expansion of size from the atom to anion.