J+J+Thomson+-+Dean+and+Harry

Sir Joseph John Thompson, OM, FRS was born on 18th September 1856 and died on 30th August 1940. Thompson was a British Physicist who was most credible for the theory of the the plum pudding model of atoms and also the discovery of electrons, isotopes and the invention of the mass spectrometer. English scientist JJ Thompson discovered the “corpuscles” (later renamed electrons) in 1897, disproving the theory at the time that atoms weren’t indivisible (the smallest particle possible). His 1904 model of the atom proposed the atoms were positively charged spheres embedded with electrons, often compared to fruit in a plum pudding. //The atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification,// was his view//.// This was disproved in 1909 with the “gold foil experiment”, when alpha particles were fired at a sheet of gold foil and most passed through unaffected.

Plum Pudding Model Thompson used experiments on “cathode rays” and found that no matter what gas he studied the same charge-to-mass ratio was obtained. He fired these rays through a type of gun, a mass spectrometer (which Thompson also invented), with magnets and plates in it, and found that when fired the rays were pulled off course toward the positive side, proving there was a negative particle in the atom. Therefore he concluded that electrons are a part of every atom. However he also knew atoms were neutral in overall charge, and so in his model proposed that the rest of the atom was positive to balance it out.

Mass Spectrometer