This number of protons is called the atomic number . The mass of the atom, its atomic mass , depends significantly upon both the number of protons and upon the number of neutrons present in the nucleus. Since the properties of the electrons depend upon the number of protons in the nucleus of an atom, each different atomic number corresponds to a qualitatively different kind of atom. Each different kind of atom makes up a different chemical element. Isotopes and Atomic Masses For many of the chemical elements there are several known isotopes .
Isotopes are atoms with different atomic masses which have the same atomic number.
Chemists sometimes find it necessary to specify the atomic mass of an isotope. This is done by writing the atomic mass as a superscript preceding the atomic letter symbol. Since uranium has the atomic number 92, a nucleus of 235 U contains 92 protons and 143 neutrons while a nucleus of 238 U contains 92 protons and 146 neutrons. Although the atomic number of an atom is implicit in the element symbol, nuclear physicists and some chemists choose to write the atomic number explicitly in nuclear reactions. The atomic number can be written as a subscript preceding the element symbol, as 92 U, but most chemists prefer to omit it since the atomic symbol includes the same information.