The part of an atom that has the most of its mass is the nucleus. The nucleus is located at the center of an atom and is composed of protons and neutrons. Protons and neutrons are much more massive than electrons, which are found in the electron cloud surrounding the nucleus.
The mass of an electron is approximately 1/1836th the mass of a proton or neutron. Therefore, the mass of the electrons is negligible compared to the mass of the nucleus. The majority of an atom's mass is concentrated in the nucleus due to the relatively large mass of protons and neutrons.