The particles responsible for the mass of an atom are primarily protons and neutrons. Protons have a positive charge, while neutrons have no charge (they are electrically neutral). Both protons and neutrons have a mass of approximately 1 atomic mass unit (amu).
Electrons, which orbit the nucleus of an atom, have a negligible mass compared to protons and neutrons. They contribute very little to the overall mass of an atom. In fact, the mass of electrons is approximately 1/1836th of the mass of a proton or neutron.
Therefore, the mass of an atom is mainly determined by the sum of the masses of its protons and neutrons, which make up the nucleus. The number of protons in an atom defines its atomic number and determines its identity as a particular element. The total number of protons and neutrons in an atom gives its mass number.