What do two different neutral isotopes of the same element have in common?
nuclear mass
nuclear number
atomic number
atomic mass

The atom of each element is made up of electrons, protons and neutrons. All atoms of the same neutral element have the same number of protons and electrons but the number of neutrons can differ. Atoms of the same element but different neutrons are called isotopes. Because of these isotopes it becomes necessary to develop a notation to distinguish one isotope from another - the atomic symbol. 

It's cell theory. easy name i would think.