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Physicist and refinement of atom model
Book - Chapter 2

In 1897 there were also experiments with strange rays called cathode rays. Joseph John Thomson (1856 —1940) researched and investigated the puzzle of these “cathode rays” from an invention of Sir William Crooke in the late nineteenth century.

He was experimenting with these cathode rays moving inside glass tubes in the Cavendish Laboratory at Cambridge University in 1897 when he discovered that the particles known as cathode rays exhibited a single charge to mass ratio. That is that their charge was related to their mass in exacting quantities. By providing a magnetic charge near the stream of particles, he observed that the cathode rays were affected by the negative poles of the magnetic field, and he called these particles “corpuscles”, and deduced that they were made of a single type of negatively charged particle.

 

In early cathode ray tubes, wire was embedded in both ends of the tube and most of the air was pumped out. Heinrich Geissler was able to evacuate more air, and then the scientists noticed light with a fluorescent glow. These cathode ray tubes are the progenitors of neon and fluorescent lighting, as well as your computer monitor and TV.
3D Model by Michael Tanzillo.

 

Thompson proposed that these “corpuscles” were smaller than atoms, or subatomic. He also suggested that they might be one constituent of all atoms. This was a bold proposal for the time, as scientists, up to that point, speculated that the atom was not divisible, but was the smallest unit of matter.

With this knowledge, Thompson created a model of the atom he called the “Plum Pudding Model”, and said the atom may be analogous to the English dessert called plum pudding, where the raisins (electrons) were dispersed throughout the positive pudding. This could be visualized, as a bowl of positive charges with electron raisins floating throughout. In discussing his discovery of the electron Thompson said:

"Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen? which itself is so small that a crowd of these atoms equal in number to the population of the whole world would be too small to have been detected by any means then known to science." [ 11]

 

Thompson’s Plum pudding model of the atom. With green spheres being the electrons, or “corpuscles”, and the surrounding grey mass a zone of positive charges.

 

In 1911, Thomson's former student, Ernest Rutherford (1871–1937) struck down this model by experimenting with different alpha particle beams, where he found evidence that the atom has a nucleus. He shot these alpha particles at ultra-thin gold foil and noticed that too many bounced back for the hypothesis of J.J. Thompson to be completely correct. Some of the alpha particles were bouncing off the protons in the nucleus of the gold atoms.

 

Alpha particles are a kind of particle that comes from radioactive nuclei. They consist of twoneutrons and two protons.

 

Rutherford’s model of the atom with imprecise electron orbit.

 

He posited that the atom may be like a small solar system but with a positively charged and large center encircled by electrons. This center was dubbed the nucleus, and it was discovered that it consisted of protons and neutrons. [12]

In 1932, James Chadwick (1891-1974) discovered a particle he called the neutron. Neutrons are also located in the nucleus and are about the same size as protons but without the electrical charge, however, they do play an important role in stabilizing the protons.

 

 

 

Bohr atom has energy shells, which the electrons can jump up to and down to. The nucleus is in red with protons and neutrons.

 

From the work of previous researchers and an atomic model posited by Gilbert N. Lewis in 1916, Neils Bohr (1885—1962) developed the planetary model of the atom. The planetary model is the model primarily taught in schools today because it is easy to understand and visualize. Bohr proposed that the chemical properties of the atom were determined by the number of electrons orbiting the atom.