Which of the following describes a cation?

• A. Positive ion, smaller than regular atoms
• B. Negative ion, larger than neutral
• C. Neutral atom
• D. Anion that loses electrons

Answer: (A)

A cation is a positively charged ion formed when an atom loses electrons. When electrons are removed, there are fewer electrons repelling each other, while the same positively charged nucleus pulls on the remaining electrons more strongly. This increases the effective nuclear attraction per electron, so the electron cloud contracts and the ion becomes smaller than the neutral atom. That’s why a description as a positive ion that is smaller than the neutral atom is correct. For example, sodium loses an electron to become Na+, which is smaller than neutral Na. The other descriptions describe either anions (negative ions), a neutral atom, or an ion that is inconsistent with being an anion, so they don’t fit a cation.