The Electron

J. J. Thomson's apparatus (photo from The Science Museum Group licensed under CC BY-NC-SA 4.0 Licence)

A tiny particle with a gigantic influence

The electron. It has a mass 1800 times less than the lightest atom, hydrogen. It is too small to even meaningfully speak of its size. Yet the movements of this particle create chemical reactions, transport energy and information, and make all our devices and appliances work. Its actions shape our society, economy, and cultures.

The electron also gave us our first peak inside the atom. By the beginning of the 20th century, the idea that matter is made of atoms was widely accepted among chemists, because it explained the proportions of substances produced or consumed in chemical reactions. But it was not yet understood what smaller parts might make up the atom; it was often thought of as a solid sphere with no internal parts. Physicists started to "shake the tree" of the atom, metaphorically speaking, to see what parts might fall out of it. The loosest hanging fruit was the electron and J. J. Thomson (1856-1940) was the shaker.

Cathode rays

The picture above shows Thomson's cathode ray tube, and below is a modern day demonstration. This is a glass tube with an electron "gun" at one end where a high voltage yanks electrons off of gas molecules and shoots them toward the other end. The electrons can be deflected to follow a curved path by applying a magnetic field or an electric field. The electrons leave a trail of light caused by collisions with air molecules. (The tube actually has a vacuum in it, but it's not a perfect vacuum. Some air molecules are needed to supply electrons to the gun, but too many air molecules would impede the journey of the electrons through the tube.)

Cathode ray tube demonstration. A magnet is used to deflect the path of an electron beam. Mateus Silva Rêgo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

But back in 1897 when Thomson was doing these experiments, some thought that the trail of light, which they called a cathode ray, was some exotic light wave rather than charged particles such as electrons. The term "electron" had only recently been coined. Thomson wanted to know: Is the beam composed of charged particles? If so, how much charge and how much mass does each particle have?

Thomson's experiment (simplified)

To understand the concept of Thomson's experiment, let's simplify it by assuming that he was shooting electrons through a perfect vacuum. They would collide with a ruled target at the other end of the tube and produce a glowing region the location of which measures the deflection of the electron beam by a magnetic or electric field.

Beam of electrons deflected downward by an electric field created by charged plates

Above is his first experiment. The electron would pass through a known electric field created between two plates by charging them with equal and opposite charges. The field would create a downward deflecting force on the electron. (Or, if you prefer, the electron is attracted toward the positively charged plate. Since opposite charges attract, the downward deflection shows that the electron has a negative charge.) But, other than the strength of the field, what determines the amount of deflection? Three factors: 1. The amount of charge (q) that the electron has; a bigger charge means a stronger force from a given field. 2. The speed (v) of the electron; the faster it moves, the less time it spends in the electric field and the smaller the deflection. 3. The mass (m) of the electron ; the larger the mass the harder it is to deflect, and the smaller the deflection. So the deflection is determined by multiplying the electric field by q/mv. If Thomson knew two of the above three factors, then he could measure the deflection and get the third one. The problem was that he did not know either the mass or the speed, although he had an educated guess about the charge from other evidence. He got the speed from an extremely clever combining of his first experiment with a second one, shown below.

Beam of electrons deflected downward by a magnetic field created by loops of electric current

He put the tube between two coils of wire, each carrying an electrical current that he could set and measure. This created a known magnetic field that would deflect the electrons downward. He then adjusted the magnetic field to get the same amount of deflection as was produced by the electric field. How does this help? A magnetic field differs from an electric field in an important way. The faster a charge moves through a magnetic field, the stronger the force. In other words, if the electron were moving extremely fast, then it would require only a very weak magnetic field to match the deflection produce by the electric field. By comparing the strengths of the electric and magnetic fields that give matching deflections, he could figure out the electron's speed. Then, knowing the speed he could get the ratio of charge to mass. If he assumed his best guess of the charge, he could get the mass.

Thomson's model of the atom

The raisin bran muffin atom

The best-guess electron mass that Thomson measured was extremely small compared to any atom. An atom, on the whole, is electrically neutral. So, the negative charge of its electrons must be balanced by an equal amount of positive charge. Thomson proposed that most of the volume inside an atom is filled with a positively charged substance, with tiny electrons embedded throughout, much like raisins in a muffin. (As Thomson's experiments were performed in Britain, plum pudding was used to make the analogy.)

The next chapter in the story is the discovery that the positive charge in the atom is highly concentrated in what is now called the nucleus. The muffin model had to be replaced by a new model in which the atom is mostly empty space.