Atomic And Ionic Radii

Atomic Radius Trends

Because the electron cloud lacks a sharp boundary, determining an atom's exact size is tricky. Instead, we estimate it using distances between bonded atoms.

For non-metals, we use the Covalent Radius (half the distance between two atoms bound by a single covalent bond). For metals, we use the Metallic Radius (half the internuclear distance separating metal cores in a crystal).

Ionic Radius and Isoelectronic Species

When an atom gains or loses electrons, its size changes dramatically. Ionic radii can be estimated by measuring the distances between cations and anions in ionic crystals.

Problem.
Which of the following species will have the largest and the smallest size?
$Mg$, $Mg^{2+}$, $Al$, $Al^{3+}$