The mass of an atom is primarily determined by its protons and neutrons, which are located in the nucleus. Protons have a mass of approximately 1 atomic mass unit (AMU), while neutrons have a slightly larger mass of about 1 AMU.
Electrons, which are negatively charged particles, have a significantly smaller mass compared to protons and neutrons. The mass of an electron is approximately 0.0005486 AMU, which is more than 1,800 times lighter than a proton or neutron.
Since electrons contribute very little to the overall mass of an atom, the majority of an atom's mass is concentrated in its nucleus, which consists of protons and neutrons. The number of protons in an atom determines its atomic number and elemental identity, while the sum of protons and neutrons gives the atom's mass number.
In summary, the protons and neutrons in an atom's nucleus are primarily responsible for its mass, while the electrons, which orbit the nucleus, contribute insignificantly to the overall mass.