The British physicist J.J. Thomson (1856-1940) was the first to offer evidence from scientific experimentation showing that the atom was not the basic and indivisible unit of matter it was previously thought to be. Thomson's experiments with cathode rays led him to discover the electron, the atom's negatively charged particle. Originally referring to his newly discovered particle as a "corpuscle," Thomson made his findings public on April 30, 1897 during a lecture given at the Royal Institution.
The first evidence for isotopes of a non-radioactive element is also credited to Thomson. He was awarded the Nobel Prize in Physics in 1906 for discovering the electron and for his work in the conduction of electricity through gases. Thomson discovered the natural radioactivity of the element potassium in 1905 and he demonstrated that the hydrogen atom contained only a single electron in 1906.
Thomson also had a theory of what was known as the "plum pudding model," which posited that electrons orbited within a sea of positive charge. This theory was later shown to be incorrect by his student, Ernest Rutherford, who described the positive charge of an atom as being concentrated in its nucleus instead.
Another one of Thomson's contributions to science was his role as a gifted teacher. Nobel Prize awards were won by seven of his research assistants. Thomson's son, George Paget Thomson, won the 1937 Nobel Prize for showing that electrons possess wavelike properties.