From the Ancient Greek atomos, that which cannot be divided. The basic constituent of matter, an assembly of fundamental particles. A particle of a chemical element which forms the smallest quantity capable of combining with others. An example: the water molecule (H2O) contains 2 hydrogen atoms and 1 oxygen atom. An atom is formed by a nucleus and a cloud of electrons 40,000 times more extensive than the nucleus itself. The latter is made up of protons, with a positive charge, and neutrons, electrically neutral.