The majority of an atom’s mass is concentrated in its nucleus. The nucleus is the central part of the atom and contains two types of particles: protons and neutrons. Protons carry a positive charge, while neutrons are neutral, having no charge.
Electrons, which are much smaller and lighter than protons and neutrons, orbit the nucleus in regions called electron clouds. Despite their presence, electrons contribute very little to the atom’s overall mass. In fact, the mass of an electron is approximately 1/1836th the mass of a proton or neutron.
To put it simply, if you were to compare the mass of the nucleus to the mass of the entire atom, the nucleus would account for over 99.9% of the atom’s total mass. This is why the nucleus is considered the ‘heart’ of the atom when it comes to mass.