Ionic Size

When atoms gain or lose electrons, the atom becomes an ion. When an atom gains an electron, it becomes a negatively charged ion that we call an anion. Anions are larger in size than their parent atoms because they have one or more additional electrons, but without an additional proton in the nucleus to help moderate the size.

When an atom loses an electron, it becomes a positively charged ion called a cation. Cations are smaller than their parent atoms because they have lost electrons (sometimes the entire outermost energy level) and the electrons that remain behind simply don't take up as much room.

Sizes of Atoms and Ions
Cations				Anions
Na 1.86	Na⁺ 0.95	Mg 1.60	Mg²⁺ 0.65	O 0.74	O^2- 1.40	F 0.71	F^- 1.36
K 2.27	K⁺ 1.33	Ca	Ca²⁺	S 1.03	S^2- 1.84	Cl 0.99	Cl^- 1.81
Values from Hein, Best, Pattison, Arena, "College Chemistry," 5th Ed., 1993, Brooks/Cole

There is a chart on the wall of the lab that shows not only the sizes of atoms, but also the sizes of ions. You might want to take a look at that. It is on the south wall of the lab. Also note that when comparing cations and anions, the anions are larger.

Practice Comparing Ionic Sizes

Try your hand at the making the following comparisons (also shown in exercise 9 in your workbook), based on your understanding of ionic size comparisons and without reference to the wall chart, except to check your answers. Answers also follow on this page.

For each of the following sets of atoms and ions, decide which is the smallest and which is the largest.

a. Na, Na⁺

b. Cl, Cl^-

c. Na⁺, Cl^-

d. H⁺, H, H^-

e. Fe²⁺, Fe³⁺

f. F^-, Ne, Na⁺