If an atom has an atomic number of 20 and is electrically neutral, how many protons, neutrons, and electrons could it potentially have (give one possible combination)?

Protons = 20, Electrons = 20. For neutrons, it could vary, for example, 20 neutrons, making the mass number 40.

Atomic number 20 means 20 protons. For neutrality, it must have 20 electrons. The number of neutrons can vary, creating isotopes. A common isotope for an element with 20 protons (Calcium) often has 20 neutrons.