Most of the mass is contained in the nucleus
The atom can be divided into two main regions, the nucleus and the electron clouds. The nucleus is home to neutrons and protons, which are the heavier subatomic particles. The mass of an atom, the atomic mass, is the sum of the number of neutrons and protons contained within the atom. The electron cloud is the region where the electrons of the atom are found. Electrons have far less mass than protons and neutrons, so they are usually neglected when considering the mass of an atom.
