Having proved that matter is made up of tiny particles called atoms, we will now learn about three key experiments in the history of chemistry that helped us understand the particles that make up atoms themselves: protons, neutrons, and electrons.
Joseph John Thompson (1856-1940) was a 1906 Nobel prize winning English physicist and renowned teacher. In fact, seven of his students went on to win their own Nobel prizes. Even Thompson’s own son was awarded a Nobel Prize in 1937.
Thomson discovered the electron while studying the properties of cathode ray tubes. A cathode ray tube is a device that produces an electric discharge when a potential is applied between two metal plates inside of a high-vacuum glass tube.
The cathode ray tube is the same technology used in an older style television set. Not a flat screen, not plasma, or LED, or OLED, but a CRT (cathode ray tube). The kind of TV your grandparents might still have in their garage or basement.
When a magnet is brought near the cathode ray, the beam is deflected. In a television, this deflection would be controlled by electromagnets that would guide the beam in a raster pattern (left to right, line by line) to produce constantly refreshing images, giving the optical illusion of motion.
J.J. Thomson noticed that no matter what metal he used in the cathode ray tube, the beam that was emitted had the exact same properties. You and I might not question this finding, but Thomson recognized its significance. He realized that if the beam of particles that was emitted from any metal had the same properties, it must be made of something that is common to all elements.
He called these particles, corpuscles. Today, we call them electrons. J.J. Thomson had discovered the electron.
Through careful measurements, he was able to determine the ratio of an electron’s charge to its mass.
He knew electrons had a negative charge, because they were emitted from the cathode in his device. He also knew that atoms were electrically neutral overall. Thus, he assumed that atoms must also contain positively charged particles. Today, we call those particles, protons.
He envisioned the electrons existing in a sea of positive charge in the atom. His idea of the atom has come to be known as the Plumb Pudding Model. It was wrong about Plumb Pudding, but it was a start!