A calcium ion (Ca²⁺) has 20 protons and 18 electrons. This is because calcium, which is the 20th element on the periodic table, typically has 20 protons and 20 electrons in its neutral state. However, when it loses two electrons to form a Ca²⁺ ion, the number of electrons decreases to 18.
The options provided suggest different configurations:
- a) 20 protons and 18 electrons – This is correct for Ca²⁺.
- b) 20 protons and 21 electrons – This would imply a neutral calcium atom with an extra electron, making it negatively charged (Ca⁻).
- c) 20 protons and 38 electrons – This suggests a heavy negative charge, which is not possible for calcium in its ionic form.
- d) 40 protons and 42 electrons – This configuration does not correspond to calcium at all.
Thus, the correct answer is option a: 20 protons and 18 electrons for the calcium ion Ca²⁺.