In an atom, the subatomic particles with very little mass are the electrons. Electrons are negatively charged particles that orbit around the nucleus of an atom. They are considered to have a negligible mass compared to the other subatomic particles present in an atom.
The mass of an electron is approximately 9.11 x 10^(-31) kilograms, which is about 1/1836 times the mass of a proton or neutron. This small mass of electrons is often disregarded when considering the total mass of an atom.
The majority of an atom's mass is concentrated in its nucleus, which is composed of protons and neutrons. Protons and neutrons are roughly equal in mass, with a mass of approximately 1.67 x 10^(-27) kilograms.