Answer: 18
Justification:
1) The most common ion of calcium is Ca 2+
2) That means that the atom, neutral calcium, has lost two electrons.
3) The atomic number of calcium is 20.
4) Atomic number is the number of protons, so calcium has 20 protons.
5) Therefore, the neutral atom of calcium also has 20 electrons (to make a net charge of zero).
6) Then, the ion Ca 2+ has 20 - 2 = 18 electrons.
Answer: 18 electrons.