J.J Thomson discovered the electron and the isotope. He was a British physicist and a Nobel laureate. He was awarded the 1906 Nobel Prize for the discoveries he had made.
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The discovery made by J.J Thompson was the electron, commonly referred to as the fundamental building block of matter. The experiments that led to the discovery are believed to have been done in the year 1897.
Joseph John Thomson is also known as J. J. Thomson. In 1897 J.J. discovered electrons. In 1904 he made the discovery of atoms and later discovered the mass spectroscope.