The charge of an electron has a negative charge (-), while protons have a postivie (+) charge.
The answer is (1) 10. The protons of Mg atom is 12. So the Mg atom has 12 electrons. The Mg2+ ion has lost two electrons so it has two positive charge. Then the answer is 10 electrons.
Atomic mass is usually defined by the total number of protons, neutrons and electrons in a single atom. You can measure it by spectrometry.
