electrons and protons
Summary Each atom of an element contains the same number of protons, which is theatomic number Z . Neutral atoms have the same number of electrons and protons. Atoms of an element that contain different numbers of neutrons are calledisotopes. Each isotope of a given element has the same atomic number but a different mass number A , which is the sum of the numbers of protons and neutrons. The relative masses of atoms are reported using the atomic mass unit amu , which is defined as one-twelfth of the mass of one atom of carbon-12, with 6 protons, 6 neutrons, and 6 electrons. The atomic mass of an element is the weighted average of the masses of the naturally occurring isotopes. When one or more electrons are added to or removed from an atom or molecule, a charged particle called an ion is produced, whose charge is indicated by a superscript after the symbol.