Answer:
A) number of electrons
Explanation:
A calcium atom has 20 electrons while a calcium ion has 18 electrons since it loses 2 electrons to form a stable octet structure. The formula of a calcium ion is Ca2+.
The number of protons does not change, neither do the number of neutrons. Atomic number is the proton number which is equals to the number of protons, hence B and C is wrong. Mass number is the number of protons and neutrons. Hence it doesnt change too and D is wrong.
Only the number of electrons change when an atom forms an ion.
