An atom is the smallest part of a chemical element.
An atom consists of an electron cloud of negatively charged electrons that surrounds a dense nucleus. This nucleus contains positively charged protons and electrically neutral neutrons. When the number of protons in the nucleus equals the number of electrons, the atom is electrically neutral; otherwise it is an ion and has a either net positive charge or net negative charge.
An atom is classified according to its number of protons and neutrons: the number of protons determines the chemical element and the number of neutrons indicates the isotope of that element.
