The particles responsible for the mass of an atom are primarily the protons and neutrons found in the atomic nucleus.
Protons have a positive charge and a relatively large mass. They contribute significantly to the mass of an atom. Each proton has a mass of approximately 1 atomic mass unit (amu).
Neutrons, on the other hand, have no charge (they are electrically neutral) but have a mass similar to that of a proton. Like protons, neutrons also have a mass of approximately 1 atomic mass unit (amu).
Electrons, which orbit around the nucleus in specific energy levels or shells, have a negligible mass compared to protons and neutrons. The mass of an electron is about 1/1836th of the mass of a proton or neutron, making it nearly 2000 times lighter.
Thus, the vast majority of an atom's mass is concentrated in the nucleus, which is composed of protons and neutrons. Electrons contribute very little to the overall mass of an atom.