An atom is the basic unit of a chemical element, consisting of a central, positively-charged nucleus which is surrounded by a cloud of orbiting negatively-charged electrons. An electrically neutral atom contains an equal number of electrons and positively-charged protons. The number of protons in the nucleus defines the chemical element to which the atom belongs and is called the atomic number. The nucleus can also contain neutrons which have no charge, and the number of neutrons determines the isotope of the element to which the atom belongs. An atom has a typical diameter of 10-11 m and the nucleus is typically 103 times smaller than the atom as a whole..