The basic building block of all normal matter, consisting of a nucleus (which is itself composed of positively-charged protonsand zero-charged neutrons) orbited by a cloud of negatively-charged electrons, so that the positive charge is exactly balanced by the negative charge and the atom as a whole is electrically neutral. Atoms range from about 32 to about 225 picometres in size (a picometre is a trillionth of a metre). A typical human hair is about 1 million carbon atoms in width.