Cation vs. Anion — What's the Difference?

Cations are typically formed when metal atoms lose electrons, resulting in a positively charged ion. For example, sodium (Na) loses one electron to become Na+. While anions are generally formed from non-metals gaining electrons, such as chlorine (Cl) gaining an electron to become Cl−. This fundamental difference in electron gain or loss reflects their contrasting roles in chemical reactions.

In terms of their role in forming compounds, cations often act as the electrostatically positive component that attracts anions, which are negatively charged, to form ionic bonds. For instance, in sodium chloride (table salt), Na+ and Cl− come together to form a stable compound due to this attraction.

Chemically, cations are usually smaller than their parent atoms because the loss of electrons reduces electron-electron repulsion in the electron cloud, leading to a decrease in radius. On the other hand, anions are larger than their parent atoms due to the addition of electrons increasing electron-electron repulsion, which expands the electron cloud.

In solutions, cations and anions behave differently based on their charge. Cations, such as Na+, tend to migrate towards the cathode (negative electrode) during electrolysis. Conversely, anions like Cl− will migrate towards the anode (positive electrode) in the same setup.

When looking at periodic trends, cations are more commonly formed by elements on the left side of the periodic table, which are metals and have a tendency to lose electrons easily. Anions, however, are more likely to be formed by elements on the right side of the periodic table, particularly the nonmetals that readily gain electrons.