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J. J. Thomson discovered the electron in 1897, using cathode ray experiments. He found that the rays were negatively charged particles, which he called electrons. He proposed the "plum pudding" model of the atom, which stated that atoms were composed of negatively charged electrons embedded in a positively charged "pudding".
Henri Becquerel discovered radioactivity in 1896, while investigating the properties of phosphorescence. He found that certain elements gave off energy in the form of radiation, which he called radioactivity.
Lord Rutherford conducted experiments in 1909, in which he bombarded a thin sheet of gold with alpha particles and observed that most of the particles passed through the gold, but a small number were deflected. He proposed a new model of the atom, in which the majority of the atom's mass and positive charge was concentrated in a tiny, dense nucleus, while the electrons orbited around it.
Dalton's model of the atom had to be modified to account for these discoveries. The discovery of electrons and radioactivity showed that atoms were not indestructible, as Dalton had proposed. The discovery of the atomic nucleus and the fact that atoms have a positive charge, showed that atoms are not even indivisible, as Dalton had proposed. The discovery of isotopes showed that atoms of the same element can have different numbers of neutrons and hence different atomic weights, but the same number of protons and hence the same atomic number.
