After studying the properties of an atom's subatomic particles, we can conclude that an atom's mass is primarily due to_____________________.

An atom's mass is primarily due to the protons and neutrons in its nucleus. 

Protons and neutrons are much more massive than electrons, which orbit the nucleus in energy shells. Electrons contribute only a tiny fraction of an atom's mass, so the overall mass of an atom is determined mainly by the number of protons and neutrons it has.