Atomic mass unit is a term used to describe the mass of a neutral atom of a nuclide.
Isotopes are Atoms of a given chemical element having the same atomic number but different atomic mass numbers. Read more.
Neutron is an electrically neutral subatomic particle with a mass approximately equal to one atomic mass unit; generally found within an atomic nucleus.
Proton is a term used to describe a subatomic particle with a charge of +1 and a mass of approximately one atomic mass unit; generally found within an atomic nucleus.
Element (chemical) is a term used to describe a substance that cannot be broken down into two or more new substances by chemical or physical processes
Atom is a neutral particle having one nucleus, an unimaginably small particle that could not be made any smaller and still behaves as a chemical system.