These species, called isotopes of the element helped resurrect Prout's hypothesis. Because now elements with fractional atomic weights could be understood as having two or more isotopes with varying atomic weights. Chlorine for example has two isotopes with atomic weights 34.98 and 35.98, resulting in an average atomic weight of 35.46. The atomic weights of each isotope however is awfully close to a whole number. But each isotope is not equally abundant.
Some isotopes are much more prevalent than others.
Hydrogen with atomic weight 1 is much more prevalent that its isotope deuterium, with atomic weight 2, or tritium with atomic weight 3. The modern notation to denote an element X , with atomic number N and atomic weight A , is . One of the earlier suggestions was that the constituents of the nucleus were protons and electrons. If an element has an atomic weight of A and an atomic number Z then, it was suggested, it had A protons and A - Z electrons. This would result in the nucleus having a positive charge of Z as required and a weight of A as needed.