A summary of the discovery of the sub - atomic particles

Photo by Norbert Kowalczyk on Unsplash

The first of the sub atomic particle to be discovered in the atom was the electron.

Physicists had learned to control and create electrical flows, however, to discover what electricity was, it required the separation of the properties of electricity from the wire or material which was impractical because when electricity flows through the wire or other material that creates an electric charge, it is impossible to separate the two. The method that could be executed in order to separate electricity from the particular materials that hold it was by studying the flow of electricity through a vacuum or near a vacuum. In 1858 Johann Geissler's invention of the improved air pump made it possible to extract sufficient air from a glass tube creating a near-vacuum in the tube. The Crookes tube contained a glass tube with a near-vacuum and a wire that carried along an electric current at one end called a cathode and at the other end, a positively charged metal plate called an anode. When a current was passed through the cathode, a thin ray of light passed on from the cathode to the anode - it was caused by heating from a small amount of gas contained in the tube by something moving through the tube, this ray of light was known as a cathode ray. It indicated that cathode rays had an electric charge due to the fact that they could be deflected by magnets and electric forces near the tube. In 1897, after measuring the velocity and amount of deflection of cathode rays, J. J Thomson found that the velocity of the cathode rays was about a tenth of the speed of light resulting in that cathode rays were not a form of electro - magnetic radiation. The deflection of the cathode rays appeared that they contain a negative electric charge. Cathode rays were not electro - magnetic energy and so the rays had to consist of particles, the particles contained a negative electric charge which meant they could not be atoms as atoms are electrically neutral. The cathode ray particles were eventually given the name: electrons.  In 1909 Robert Millikan separately measured the charge of the electron which helped him to calculate the mass of the electron and concluded that it was a particle extremely smaller than the atom.

Atoms are electrically neutral so if the electron was a negatively charged particle within the atom there had to be a positively charged part within the atom so the charges could cancel out. Another question was concerning how the electron and the positively charged material within the atom were arranged.      J.J Thomson was aware that atoms contained a negative charge, thereafter, he reasoned that there should be a positive charge within the atom to cancel out the negative charge on the electrons - this led him to suggest that the atom was structured as just as a plum pudding, with the majority being the positively charged matter and the negatively charged electrons embedded in the positively charged matter, like raisins are embedded in a plum pudding. 

New Zealand Physicist Ernest Rutherford discovered the structure of the atom and the positively charged material in the atom in 1911, in his experiment he fired alpha particles at a thin sheet of pure gold foil, and thereafter measured the effect on the alpha particles as they scattered by deflection by the gold foil. Alpha particles are helium nuclei, and they had been discovered as one of the three types of rays emitted by radioactive elements, while the other two rays were beta particles and were later discovered to be electrons and gamma rays which is a form of electromagnetic energy of a particular wavelength. In 1896 radioactivity had been discovered by French physicist Becquerel and was exclusively studied and researched by Pierre and Marie Curie, they isolated a radioactive element known as radium which Rutherford used in his experiments, he used radium as the source of alpha particles when the particles were fired at sheets of gold foil. In 1907, Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford began an experiment on the scattering of alpha particles when they were fired at thin metallic foils. A sample of radium was placed in a lead box that contained a small pinhole in it, alpha particles emitted from radium - passed through the pinhole in the lead box and then hit a gold foil which resulted in some deflection of the alpha particles as they were affected by the atoms in of the gold foil. As the alpha particles hit a zinc sulphide screen their deflection was measured as they made a flash of light. Rutherford predicted based on Thomson's plum pudding model that the majority of alpha particles would directly pass through the gold foil because the positive charge in the plum pudding model was presumed to be spread out throughout the volume atom, therefore the electric field from the 'plum pudding' would be too weak to affect the relatively mass and fast-moving alpha particles. However, to their extreme surprise, it was discovered that a few of the alpha particles deflected backward more than 90 degrees from their path. Rutherford described the results :  

"It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

Therefore, the plum pudding model did not make sense hence, a new atomic model was needed. Analyzing the results of this experiment, Rutherford used it to develop a new model for the atom. The model suggested a nucleus in the center with a positive charge, this positively charged nucleus caused the strong backward deflection of the positively charged alpha particles. Negatively charged electrons surrounded the nucleus however, as the majority of alpha particles passed through the gold foil without any deflection, it made sense that most of the atom was empty space and that electrons were at some distance from the nucleus. 

The discovery of the nucleus brought many questions as to why the negatively charged electrons did not fall over into the positively charged nucleus as generally opposite charges attract each other and why the positively charged nucleus in all atoms with two or more protons (every atom except the hydrogen) did not fly apart as same charged particles repel each other. Thereafter Neils Bohr put forward that if electrons were in constant motion around the nucleus in specific orbits they would not fall into the nucleus due to centrifugal force. Electrons could jump from one orbit to another and could not occupy any position in between the orbits. The reason why nuclei of two or more protons did not fly apart was due to the existence of the strong force that holds the nucleus together. The strong force is from protons exchanging special particles called pions, a process capable of generating an attractive force between protons.

However, for atoms other than the hydrogen atom, their atomic weight seemed to be at variance with their atomic number. The atomic number is of such importance as it shows the number of protons and electrons an atom has. The atom hydrogen has a single proton and electron thereafter its atomic number is 1 and its atomic weight is 1. As another example, Helium has 2 protons, and because it is normally electrically neutral, it contains 2 electrons and its atomic number is 2 yet the atomic weight is slightly more than 4 and as the majority of the weight in an atom is contained in the nucleus its atomic weight would be expected to be two, rather it was slightly more than 4. Atoms of the same element were discovered to have different weights using the process known as a mass spectrograph. Ions, of the same element (atoms that have either lost or gained electrons) and, are electrically charged were sent through a glass tube containing a vacuum in an experiment. Ions (being electrically charged) are sent in a stream and would be deflected by the presence of a magnet and was observed that the stream of ions would break into several different streams, which put forward that ions had different weights. Lighter ions were deflected more than heavier ions, it was put forward that the reason why the ions stream had divided into several different streams was that the original stream consisted of ions of various different weights. Therefore atoms of the same element with different weights were called isotopes, but the reason why atoms of the same element had different weights was unknown. 

The clearest explanation for these problems was that there was more matter in the nucleus than just the protons. In 1920, Rutherford put forward that there must be some undiscovered electrically neutral matter within the nucleus to be the reason why the atomic weight of the atom (other than hydrogen) were more than double their atomic number and why atoms of the element could have different weights. 

In 1930, Walther Bothe and Herbert Becker performed an experiment that was further approved by Irene and Frederic Joliot - Curie. They, however, unfortunately, misinterpreted their results and believed to have detected y-rays while they had actually seen neutrons. Furthermore, after additional experimental verification, James Chadwick discovered the correct interpretation of the following experiments in 1932. Instantly the new particle, the neutron became an essential performer of nuclear and elementary particle physics, additionally, it completely changed the whole research. 

Enrico Fermi and his group applied it to artificial radioactivity, and substituted neutrons for a - rays that were initially used by Joliot - Curies. On top of that, they discovered that slow neutrons were more efficient than fast ones in certain nuclear reactions. Otto Hahn, Fritz Strassmann, Lise Meitner and Otto Frisch discovered after many misinterpretations of advanced experimental results what is called nuclear fission. Joliot, Halban, and Kowarski showed the possibility of a chain reaction by neutron multiplication due to fission, nuclear physics became a military science at the time when the Second World War was beginning. Later on, it lead to nuclear power applications and the use of neutrons became an important tool and object of scientific research at large-scale neutron facilities.

In conclusion : 

The standard model of the atom had the proton and neutron making up the nucleus alongside electrons orbiting the nucleus in particular orbits. The discovery of the neutron cleared the confusion as to why the atomic weight of an atom could vary from its atomic number and how different isotopes of an atom could have different weights. 

$1$