(b) Ionic Radius

(b) Ionic Radius

Cations and Anions: Size Relative to the Parent Atom

When an atom loses an electron to form a cation (a positive ion), its size decreases. The nuclear charge remains the same, but there are now fewer electrons. The remaining electrons are pulled more strongly by the nucleus, causing the ion to contract. For example, the atomic radius of sodium (Na) is 186 pm, but the ionic radius of $\text{Na}^+$ is only 95 pm.

When an atom gains an electron to form an anion (a negative ion), its size increases. The nuclear charge remains the same, but there are now more electrons. The increased electron-electron repulsion causes the electron cloud to expand, and the effective nuclear charge felt by each electron decreases. For example, the atomic radius of fluorine (F) is 64 pm, but the ionic radius of $\text{F}^-$ is 136 pm.

The Trend for Isoelectronic Species

Isoelectronic species are atoms and ions that contain the same number of electrons. For example, $\text{O}^{2-}$, $\text{F}^-$, $\text{Na}^+$, and $\text{Mg}^{2+}$ all have 10 electrons. Even though they have the same number of electrons, their radii are different because they have different nuclear charges.

The rule is simple: For isoelectronic species, the greater the nuclear charge (the more protons), the smaller the radius. A higher positive charge pulls the same number of electrons in more tightly. Conversely, an anion with a greater negative charge will have a larger radius because the net repulsion among the electrons outweighs the nuclear pull. …