Electrons are particles of the atom, just as atoms are substructures of matter. Some of the basic features of atoms were revealed through the experiments that were used in the discovery of the electron. Since the electron was the first subatomic particle discovered, it is seen as one of the turning points in physics. In today's chemistry and physics, it turned out to be one of the most important ones for explaining the chemical bond.
JJ Thomson and the discovery of the electron
Gas discharge tubes today are mostly called cathode-ray tubes because the rays originate at the cathode. Even though other scientists experimented with cathode-ray tubes, JJ Thomson improved and expanded the scope of experiments. With both magnetic and electric fields, he confirmed the negative charge of cathode rays.
He also used a metal cup to catch the rays and discovered an overabundance of negative charge. Thomson was also able to determine the charge of the electron in relation to its mass, qe / me, which was a crucial step in determining the exact values of both qe and me.
A cathode ray tube. flickr.com
Elements of the electron discovery process
The elements of the electron discovery process were: the discovery of the electrons, the plum pudding model, Ernest Rutherford, and the gold foil experiment, and the Millikan oil-drop experiment. These steps are important not just because of the discovery of the electron but also because they shape the electron model we have today.
Discovery of the electrons and the plum pudding model
In addition to discovering electrons by experimenting with cathode-ray tubes, JJ Thomson also studied positively charged particles in neon gas. Thomson discovered that the conventional atom model failed to account for negatively and positively charged particles. As a result, he presented a plum pudding model of the atom. In this model, he claimed that:
The raisins in the pudding were represented by negative electrons, while the positive charge was held by the dough.
All atoms include electrons, which are small negatively charged subatomic particles.
Negative particles are floating around in a positively charged soup in an atom.
Atoms have an overall neutral charge.
Fig. 1. JJ Thomson's plum pudding model of the atom.
Discovery of the electron: Ernest Rutherford and the gold foil experiment
In 1909 Rutherford carried out the gold foil experiment in which he discovered the nuclear nature of atoms by deflecting alpha particles through a thin gold foil. Rutherford's idea was to look for alpha particles with very high deflection angles, angles that were not expected from any theory of matter at the time.
Hans Geiger and Ernest Marsden conducted the experiment in 1909, directing an alpha-particle (helium nuclei) beam towards thin gold foil and then measuring the angle of Rutherford's scattered alpha particles as a consequence of these particles scattering off gold foil atoms.
They utilized an alpha particle detector that produced fluorescent light spots when alpha particles struck it. The experiment included radon, radium, and bismuth alpha particle emitters.
These deflections could only happen if a large portion of the positive charge was located at the nucleus. It was the data he collected from this experiment that led him to his atomic model in which he claimed that:
- Atoms are mostly empty space, with the positive charge highly located in the center: the nucleus.
- The nucleus contains the bulk of the atomic mass.
- The nucleus has lighter and negatively charged electrons around it.
Fig. 2. Ernest Rutherford at McGill University in 1905, working on his experiment. flickr.com
Discovery of the electron: Millikan oil-drop experiment
The Millikan oil drop experiment by Robert Millikan took place in 1909. It resulted in the first precise direct measurement of a single electron's charge, one of nature's most fundamental constants. In his experiment, he saw that when fine oil drops are sprayed, they get charged.
The charge on a drop may be calculated using the balance of gravity and electric forces. The charge is discovered to be quantized in units of \(−1.6 \cdot 10 ^ {−19} C\) allowing the charge of the excess and missing electrons on the oil droplets to be determined directly.
Discovery of the electron - key takeaways
- In 1897 JJ Thomson discovered the electron by using cathode-ray tubes.
- Three big experiments that helped shape the electron and the atom concept that we have today are the discovery of the electron and the plum pudding model, Rutherford's gold foil experiment, and the Millikan oil-drop experiment.
- The gold foil experiment is one of the most important experiments since it led to the discovery of the nucleus.
- Millikan's oil-drop experiment resulted in the first precise direct measurement of a single electron's charge.
- The discovery of the electron is a turning point in physics because it was the first subatomic particle discovered. It turned out to be the most crucial for describing the chemical bond in today's physics.
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