Question

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Answer 1

protons = atomic
atomic weight >> look at the periodic table
neutrons = atomic weight-protons approximately (electron mass is negligible in most cases)
ionic charge-- if you are referring to a magnesium ion, 2+, it has 2 valence electrons it will give away to become neutral.

Answer 2

Sorry, ignore my last line. By giving away two electrons, it becomes Mg 2+ :P

Answer 3

So it would be protons?