Which of the following statements is correct?

Step 1: Analyze the question We need to find the one true statement about atomic and ionic size (periodic trends).

Step 2: Key concepts

• Down a group: more electron shells are added, so atomic size increases.
• Across a period (left to right): nuclear charge increases while electrons enter the same shell, so the nucleus pulls the electrons in more strongly and atomic size decreases.
• Cations: formed when an atom loses electrons. Often the whole outer shell is lost, and the remaining electrons are pulled more strongly by the nucleus (more protons than electrons). So cations are smaller than the parent atom.
• Anions: formed when an atom gains electrons. Electron-electron repulsion increases while nuclear charge stays the same, so anions are larger than the parent atom.

Step 3: Check each option

• A. Atomic size decreases down the group. ❌ It increases.
• B. Atomic size increases across the period. ❌ It decreases.
• C. Anions are smaller than the parent atom. ❌ They are larger (e.g., Cl⁻ is larger than Cl).
• D. Cations are smaller than the parent atom. ✅ Correct (e.g., Na⁺ is smaller than Na, since Na⁺ has 2 shells while Na has 3).

Step 4: Answer

Tip: Remember "Cations are Cute (small), Anions are Awkwardly big." Also: size goes up going down a group, and down going across a period.