The mass of an atom is primarily concentrated in its nucleus. The nucleus is the central part of an atom and is composed of protons and neutrons. Protons have a positive charge, while neutrons have no charge. The number of protons in the nucleus determines the atomic number of an element and defines its identity. Neutrons provide stability to the nucleus by balancing the repulsive forces between protons.
The nucleus is incredibly dense compared to the rest of the atom. The mass of a proton or neutron is approximately 2,000 times greater than that of an electron. Electrons, which have a negative charge, are much smaller and lighter compared to the nucleus. They orbit the nucleus in regions called electron shells or energy levels.
In summary, the vast majority of an atom's mass is concentrated within its nucleus, while the electrons occupy the relatively larger volume surrounding the nucleus.