Discovery of the Electron

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.