A calcium atom and a calcium ion have the same

Analyze the question: We are comparing a calcium atom (Ca) with a calcium ion (Ca²⁺). We need the property that stays the same when the atom becomes an ion.

Concept: When an atom forms an ion, it gains or loses electrons only. The nucleus (protons and neutrons) is unchanged.

Calcium atom vs. calcium ion:

• Calcium atom: 20 protons, 20 electrons, 20 neutrons (for Ca-40), electronic structure 2, 8, 8, 2
• Calcium ion (Ca²⁺): 20 protons, 18 electrons, 20 neutrons, electronic structure 2, 8, 8

Checking each option:

• A. Chemical properties: different. Ca is a reactive metal that reacts with water. Ca²⁺ is a stable ion with different behaviour. ✗
• B. Electric charge: the atom is neutral, the ion is +2. ✗
• C. Electronic structure: 2,8,8,2 vs 2,8,8. ✗
• D. Number of electrons: 20 vs 18. ✗
• E. Number of neutrons: both have 20 (same nucleus). ✓

**Answer: E.

Tip: Ion formation involves only electrons. Protons and neutrons stay the same, so anything about the nucleus (the number of protons, neutrons, and mass number) is unchanged. Anything about electrons, charge, or chemical behaviour changes.