What's Atom?

The smallest unit of an element is known as atom. Atom is the basic unit of a substance. An atom is made up of three sub-atomic particles including proton, electron and neutron. Neutrons and protons are present in the nucleus of an atom and electrons revolve around the nucleus in fixed energy levels or orbits. Electron has negative charge and proton has positive charge. Neutron has no charge therefore, the charge on a nucleus is positive. The whole mass of an atom is present at the center of an atom and it depends on the number of protons and neutrons present in the nucleus. The mass of an electron is negligible as compared to the mass of proton and neutron so, it does not contribute in the mass of an atom.