Magnesium combines with oxygen to form magnesium oxide. if 24.46 g of magnesium reacts completely with 5.41 g of oxygen, what is the percent by mass of oxygen in magnesium oxide?

The magnesium oxide is MgO. The mass percent of oxygen is 16/(24.3+16)=39.7%. For the given reaction, in my opinion, there will be magnesium left after reaction. 

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.