Discovery of the Electron
Previously, we studied JJ Thomson's cathode ray tube experiment. He discovered the electron in 1897. It was a massive scientific leap. Before Thomson, scientists presumed Dalton's atomic theory. Dalton was mostly correct. For example, elements have the same types of atoms. So, a chunk of pure gold consists of only gold atoms. He also realized that compounds are made of different atoms in whole-number ratios. But, he assumed that the atom was the smallest fundamental unit. With Thomson's discovery, we knew there were subatomic particles. Atoms had smaller building blocks! It would be many years before the discovery of the proton and neutron. But, he had opened up a new area of exploration in physical chemistry! What are the properties of the electron?
Thomson took the first step. From the cathode ray tube experiment he calculated the electron's charge-to-mass ratio. He determined it was 1.78×108gC. The units are coulombs per gram. The coulomb is the SI unit for a charge. But, a charge-to-mass ratio is only so helpful. What's the actual charge? What's the actual mass? There was still more work to do!
An Experiment to Find the Electron's Charge
Robert Millikan was the first scientist to develop an experiment to determine the electron's charge. He performed his famous oil drop experiment in 1909. The experimental setup was quite ingenious. He used a spray bottle to create aerosolized oil droplets. The oil got sprayed above a metal plate. The plate had a tiny hole. So, a few droplets passed through it. Below was a second plate. He connected the two plates to a voltage source. The voltage caused the upper plate to build a positive charge. The lower plate developed a negative charge. When the oil was between the plates, he would ionize them using x-ray radiation. The nitrogen, oxygen, and hydrogen, in the air, would lose electrons and transfer them to the oil. So, the droplets held negative charges.
First, he measured the velocity of the droplets without any voltage. At this point, he applied some knowledge from physics. At terminal velocity, the drag force equals the gravitational force. Both are related to the droplet's radius. That allowed him to calculate the volume. Then he used the oil's density to convert volume to mass. So, he knew the mass of the droplet.
Next, he turned on the voltage source. Remember, the droplets were ionized. So, they have an ionic charge. So, they experienced electrostatic interactions from the metal plates. He fine-tuned the voltage until the droplets became suspended in the air. That meant the gravitational force and the electric field were equal. Based on the electric field, he calculated the charge of the oil droplet. He compared droplets of various sizes. He observed that their charges were whole number integers of a specific value. He assumed that there could only be whole numbers of electrons. That's an excellent assumption because you can't have half an electron. So, he calculated a single electron's charge as −1.59×10−19C.
Calculating the Electron's Mass
Robert Millikan had determined the electron's charge! That's a huge accomplishment. With the charge, we can calculate the electron's mass. How? We can use the mass-to-charge ratio from the cathode ray tube experiment.
−1.78×108C1g×e−−1.59×10−19C=8.93×10−28e−g
Modern Values
Are you wondering how close Millikan and Thomson were to our modern values? The current mass value is 9.10938×10−28g. That's really close to the result we just calculated! What about the charge? Our best result is 1.60217663×10−19. Millikan was within one percent! That's not too bad for measuring oil drops! The difference from our modern value is caused by experimental error and uncertainty.
