Isotopes

      One of the flaws in Dalton's five points about chemistry is that all atoms of a single element are identical. The number of protons in the nucleus will be the same, but the number of neutrons can vary among atoms of the same element. The various amounts of neutrons in the nucleus, which have a neutral charge, create various isotopes of an element. These isotopes will also vary in mass as well as neutrons.

                                                http://www.ducksters.com/science/chemistry/isotopes.php
       To name an isotope, you must use isotope notion. It displays the symbol of the element, the mass number (sum of protons and neutrons) on the upper left, and the atomic number (number of protons) on the bottom left.

http://thechemguys.blogspot.com/2011/04/april-18-atomic-mass-and-isotopes.html