J.J. Thomson announced the discovery of the electron in 1897, and in 1906 he was awarded the Nobel prize for his research. In his research he concluded that the atom contains a small but dense nucleus, and that this nucleus must have a positive charge to balance the negative charge of the electrons. Introduction to the Modern Concept of Atomic Structure. An atom usually has the same number of protons, neutrons and electrons in order to maintain a balance. The difference between an atom of one element and that of another element is in the number of protons.
Isotopes are atoms of the same element with a different number of neutrons.
They differ only in their number of neutrons. Physically, tritium is the heaviest because it contains the heaviest nucleus. Chemically they are all identical. A-Z 4.8 Introduction to the Periodic Table. Elements are organized in the Periodic Table by their atomic number Z , that is, by their number of protons.