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. . . was a British physicist that discovered the first subatomic particle, the electron, in 1897. observed that the beam in a gas discharge tube was affected by a magnetic field - it was deflected by the negative pole and attracted to the positive pole. From his initial observations and subsequent detailed measurements, Thomson concluded that . . . .

1. The beam (and particles making up the beam) are negatively charged.
2. The particles have significantly less mass than atoms.
3. The particles are the "same" regardless of the source material.

Thomson calculated the charge-to-mass ratio of the particle as 1.759 × 10¹¹ Coulombs/kg. While Thomson is credited with the discovery of the electron, the physicist George Stoney was the first to use the name "electron".

. . . was an American experimental physicist that determined the charge of an electron and an accurate value of Planck's constant. determined the charge of an electron with his famous Video of Millikan's Experiment In the experiment, a microscopic oil droplet with an electron on its surface was suspended in air with an applied electric field. From the experimental data, Millikan calculated the charge of an electron (1.5924 × 10^-19 C). Dividing the charge (in Coulombs) by Thomson's charge-to-mass ratio (1.759 × 10¹¹ Coulombs/kg) gave the mass of the electron as 9.053 × 10^-31 kg (the accepted value is 9.107 × 10^-31 kg).

. . . was a New Zealand experimental physicist that investigated the structure of the atom and the chemistry of radioactive elements. aimed a beam of . . . the nucleus of a Helium atom. It contains 2 protons and 2 neutrons. It has a +2 charge and a mass of 4 amu. at a thin piece of gold foil in what is known as the Video of Rutherford's Experiment Rutherford made a remarkable observation - a small percentage of the particles were deflected from their straight-line path and an even smaller number bounced back toward the source. Rutherford concluded that . . . .

1. Most of the atom's volume is empty space.
2. A positively-charged nucleus caused the deflections.
3. In rare instances, a small, relatively heavy nucleus was struck by the much lighter alpha particle causing it to bounce backwards.
4. Rutherford discovered the proton.

How did Rutherford know Thomson had previously discovered the negatively charged electron, so there must be a positively charged particle to make the overall charge of an atom equal zero. In addition, the path of a positively-charged α particle will be altered (deflected) by a positively-charged nucleus ("like charges repel").