Cheetham Hill, near Manchester, England—died August 30, 1940, Cambridge, Cambridgeshire) was an English physicist who helped revolutionize the knowledge of atomic structure by his discovery of the electron(1897). He received the Nobel Prize for Physics in 1906 and was knighted in 1908. Thomson determined that all matter is made up of tiny particles that are much smaller than atoms. He originally called these particles 'corpuscles,' although they are now called electrons. This discovery upended the prevailing theory that the atom was the smallest fundamental unit.

In 1906, Thomson began studying positively charged ions, or positive rays. This led to one of his other famous discoveries in 1912 when he channeled a stream of ionized neon through a magnetic and an electric field and used deflection techniques to measure the charge to mass ratio. In doing so, he discovered that neon was composed of two different kinds of atoms, and proved the existence of isotopes in a stable element. This was the first use of mass spectrometry.

• Best Known For: J.J. Thomson was a Nobel Prize-winning physicist whose research led to the discovery of electrons.

Thomson held that atoms are uniform spheres of positively charged matter in which electrons are embedded. Popularly known as the plum pudding model, it had to be abandoned (1911) on both theoretical and experimental grounds in favour of the Rutherford atomic model, in which the electrons describe orbits about a tiny positive nucleus.

A plum pudding model is a historical scientific model of the atom that was proposed by J.J. Thomson in 1904, shortly after he discovered the electron. The model tried to explain two properties of atoms that were known at that time: electrons are negatively charged particles, and atoms have no net electric charge.

This discovery was pivotal in advancing atomic theory, as it provided crucial evidence that electrons are fundamental particles of nature with a consistent, unchangeable charge. Millikan’s experiment also helped refine the understanding of atomic structure and contributed to the development of quantum mechanics.

Additionally, his findings helped confirm Robert A. Millikan's earlier work on Planck's constant and energy quantization in photoelectric effect studies, further supporting emerging quantum theories. For this work, Millikan was awarded the Nobel Prize in Physics in 1923.