atom
The smallest stable unit of a chemical element. An atom is typically 0.1 nanometre across. Nearly all the mass is concentrated into a positively charged nucleus, which is a thousand times smaller than the atom as a whole. In a neutral atom the nucleus is surrounded by a cloud of electrons, the number of which matches the positive charge of the nucleus, and is termed the atomic number. Each element has a different atomic number, starting with 1 for hydrogen, 2 for helium, and so on.
The electrons can adopt any one of a set of discrete energy levels within an atom. The set of energy levels available is unique to each element. If the electrons make a transition from one energy state to another, electromagnetic energy is either absorbed or emitted. Such transitions result in the production of absorption line spectra or emission line spectra.
If sufficient energy is absorbed by an atom, one or more electrons may be torn away completely in the process called ionization. Energy for ionization can be provided either by electromagnetic radiation or by the thermal energy available in a hot gas. An ionized atom carries a positive electric charge. Any ion with one or more electrons has its own energy levels and spectrum, which are not the same as those of the parent atom.