The subatomic particle that provides mass to the atom is the neutron. Neutrons, along with protons, reside in the atomic nucleus and contribute to the overall mass of an atom, while electrons, which are much lighter, orbit around the nucleus. The number of neutrons in an atom can vary, leading to different isotopes of an element. Neutrons play a crucial role in stabilizing the nucleus and influencing the atom's nuclear interactions, making them vital in nuclear physics and chemistry.