Magnesium is an element with atomic number #12#. This means that every magnesium atom will have #12# protons.
In a magnesium atom, there are #12# electrons, to make the atom have a neutral charge. When an ion is formed, the magnesium atom loses electrons. The #2+# charge shows that the magnesium ion has two more protons than electrons. The proton number for magnesium does not change, so #"Mg"^(2+)# must have two less electrons, i.e. #10# electrons.
The number of neutrons varies among different isotopes of magnesium. An isotope of an element means it just has different amounts of neutrons.
The most common isotope of magnesium is #""^24"Mg"#. This #24# indicates the total number of protons and neutrons. Magnesium has #12# protons, so must have
#24 - 12 = "12 neutrons"#
Other isotopes of magnesium have different numbers of neutrons, i.e. #""^25"Mg"# has #13# neutrons, #""^26"Mg"# has #14# neutrons. #""^24"Mg"# is most likely being referred to in the question, though.
I hope this helped!
