Question

If an atom has an atomic mass of 25 and has 12 protons, how many neutrons does the atom have?

Answer 1

the atomic mass is the sum of the protons and neutrons

Answer 2

So as @UnkleRhaukus said, the atomic mass is the combination of the neutrons and protons masses - since an electron is negligible in mass. If you have 12 protons, then of the mass that is 25, 12 of that is protons. How much is left for neutrons?

25-12 = 13. This means that there should be 13 neutrons in the atom.

Magnesium is the element, by the way, with 12 protons.

Answer 3

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