In a neutral atom the number of electrons equals the number of protons. When ionisation occurs, one or more electrons are provided with enough energy to escape the atom altogether. What remains is known as an ion because it is now positively charged and will behave in a different way chemically from neutral atom. There are just over 110 known chemical elements and these can be arranged in a classification sequence known as the periodic table . The number of neutrons in any given atomic nucleus can also vary.
Most atoms have a prefered number of neutrons at which their nuclei are stable.
Atoms with more or less neutrons than the stable configuration are known as isotopes and will often undergo radioactive decay. The simplest of all atoms is hydrogen and is a special case which has a nucleus of one solitary proton and a single electron in orbit around it. See also atomic absorption, atomic emission, atomic number, molecule. Atomic absorption . The process by which atoms absorb certain, prefered wavelengths of electromagnetic radiation.