If an isotope of an element has 27 neutrons and a mass number of 52, how many electrons must it have?

25 Since you have a mass number of 52, you know that the sum of the protons and neutrons is also 52. And since you know that there's 27 neutrons, then the number of protons has to be 52 - 27 = 25 Assuming that the atom has a neutral charge, then the number of electrons has to match the number of protons. So the number of electrons will be 25.