Joseph John Thomson made several discoveries that helped revolutionize the understanding of atomic structure. Thomson received the Nobel Prize in physics in 1906 for his experiments examining discharges of electricity in gases. Thomson is credited with identifying electrons as particles of an atom, and his experiments with positive-charged particles led to the development of the mass spectrometer.
TL;DR (Too Long; Didn't Read)
In the late 1890s, physicist J.J. Thomson made important discoveries about electrons and their role in atoms.
Thomson's Early Life
Thomson was born in a suburb of Manchester, England, in 1856. He did well in school, and his professor of mathematics suggested that Thomson apply for a scholarship at Trinity College at Cambridge. Thomson went on to become a Fellow of Trinity College in 1880. He was a professor of experimental physics and launched an attempt to build mathematical models to explain the nature of atoms and electromagnetism.
Experiments with Electrons
Thomson's most celebrated work came out experiments he conducted in 1897 at his Cavendish Laboratory at Cambridge University. He identified particles in cathode rays in a vacuum tube and correctly postulated the rays were streams of particles that were contained in atoms. He called the particles corpuscles. Thomson was correct about the existence of the particles, but these negatively charged particles are now known as electrons. He demonstrated a device that "steered" the path of electrons with electric and magnetic fields. He also measured the ratio of the electron's charge to its mass, which led to insights about how light the electron is compared to the rest of an atom. Thomson received the Nobel Prize for this groundbreaking work.
Discovery of Isotopes
In 1913, Thomson continued his experiments involving cathode rays. He focused his attention on canal, or anode, rays, which are beams of positive ions created in certain types of vacuum tubes. He projected a beam of ionized neon through magnetic and electrical fields and then measured how the beam deflected by passing it through a photographic plate. He discovered two separate patterns for the beam, which indicated two atoms of neon with different masses, better known as isotopes.
Invention of Mass Spectrography
Thomson had hit on a process to measure the properties of atomic masses. This process led to the development of the mass spectrometer. Francis William Aston, one of Thomson's students, continued the research and built a functioning mass spectrometer. Aston went on to win a Nobel Prize in chemistry for his work identifying isotopes.
Legacy: Fundamentals of Physics
Although many other scientists made observations of atomic particles during the time of Thomson's experiments, his discoveries led to a new understanding of electricity and atomic particles. Thomson is rightfully credited with the discovery of the isotope and the invention of mass spectrometers. These accomplishments contributed to the evolution of knowledge and discovery in physics that have continued to the present.