Discovery of Electron, Proton and Neutron for IIT JEE

Cathode Ray Experiment JEE - Electron, Proton and Neutron

Dalton’s atomic theory was able to explain the law of conservation of mass, law of constant composition and law of multiple proportions very successfully. Three different kinds of sub-atomic particles were discovered in the nineteenth and the twentieth century, namely electron, proton and neutron. Each of these sub atomic particles involved different set of experiments which were conducted by different scientists to prove the existence of these particles. These are described, in detail, as follows:

Discovery of electron

In 1885, using cathode ray tubes, Sir William Crookes conducted a series of experiments to study the behavior of metals heated in a vacuum. A cathode ray tube consists of two metal electrodes in a partially evacuated glass tube. An evacuated tube is the one from which most of the air has been removed. Cathode is called the negatively charged electrode, while anode is called the positively charged electrode. These electrodes are connected to a high voltage source. Such a cathode ray tube has been shown in the following figure.



It was observed that the cathode produced a stream of particles when very high voltage is passed through the electrodes in the evacuated tube. It has been shown that these particles travel from the cathode to the anode and have been called cathode rays. In the absence of external magnetic or electric field, these rays travel in a straight line. In 1897, an English physicist Sir J.J. 
Thomson showed that the rays were made up of a stream of negatively charged particles. This conclusion was drawn from the experimental observations in the presence of an external electric field when the experiment was conducted. The important characteristics of cathode rays are as follows: 

  • • Cathode rays travel in a straight line

  • • Cathode ray particles carry mass and possess kinetic energy

  • • The particles that constitute the cathode rays have insignificant mass but they travel very quickly

  • • By applying an external electrical field, cathode ray particles carry negative charge and are attracted to positively charged plate.

  • • The nature of the cathode rays produced was independent of the nature of the gas filled in the cathode ray tube and the nature of the metal used for cathode and anode production. The charge to the mass ratio (e / m) has been found to be the same in all cases.

  • These particles that make up the rays of the cathode were later called electrons. The nature of cathode rays was observed to be the same regardless of the metal used for the cathode or the gas filled in the cathode ray tube. This led Thomson to conclude that electrons must be present in all atoms. This meant that, as Dalton and others believed, the atom is not indivisible. In other words, we can say that the atomic structure theory of the Dalton partially failed. This conclusion raised a question that if the atom was divisible, then what and how many were its constituents? Today a number of smaller particles are found to constitute atoms. These particles constituting the atom are called subatomic particles. After this discovery, we came to know that electron is one of the constituents of the atom. One year later, the world came to know about another constituent particle present in an atom – proton. As the atom is neutral, there is a presence of positively charged particles in the atom so as to neutralise the negative charge of the electrons.

    Discovery of proton

    Eugen Goldstein (1886) carried out an experiment using a perforated cathode (a cathode with holes in it) in the discharge tube filled with air at a very low pressure long before the discovery of electron. When a high voltage was applied across the electrodes in the discharge tube, a faint red glow was observed behind the perforated cathode, as shown in the following figure.



    This glow was due to another type of rays flowing in the opposite direction to that of the cathode rays. These rays have been called as rays of anode or positive rays. These have been charged positively and have also been called canal rays because they have passed through the holes or canals present in the perforated cathode. The following observations have been made on anode rays (canal rays): the anode rays also travel in straight lines like cathode rays. The anode particles carry mass and have kinetic energy. Canal ray particles are much heavier than electrons and carry positive charges. The positive charge on the particles was whole number multiples of the amount of charge present on the electron. The nature and type of the particles making up the anode rays depended on the gas in the discharge tube. In terms of the interaction of cathode rays with the gas present in the vacuum tube, the origin of anode rays can be explained. It can be explained as follows: the electrons emitted from the cathode collide with the gas's neutral atoms in the tube and remove one or more electrons in them. This leaves the positive charged particles traveling to the cathode behind. When the cathode ray tube contained hydrogen gas, the obtained channel ray particles were the lightest and the highest was their weight to weight ratio (e / m ratio). Rutherford showed that the hydrogen ion (hydrogen atom from which one electron was removed) was identical to these particles. These particles have been named protons and have been demonstrated to be present in all matter. Thus, we see that the experiments by Thomson and Goldstein had shown that an atom contains two types of particle which are oppositely charged and an atom is electrically neutral. In addition to the two charged particles namely the electron and the proton, a neutral particle called neutron was also discovered years later about which is described, in detail, in the following section.

    Discovery of Neutron

    James Chadwick has been a war prisoner in Germany for four years. When the First World War came to an end, he returned to his native England to join Ernest Rutherford, the mentor of his graduate days. Now head of the nuclear physics laboratory at Cambridge University, Rutherford oversaw Chadwick's PhD in 1921 who then became the laboratory's assistant director. Radioactivity was the focus of Chadwick's own research. Rutherford had discovered the proton in 1919, a particle positively charged within the nucleus of the atom. But the proton did not seem to be the only particle in the nucleus. As they studied atomic disintegration, they continued to see that the atomic number (the number of protons in the nucleus, equivalent to the atom's positive charge) was lower than the atomic mass (the atom's average mass). For example, there is an atomic mass of 4 in a helium atom, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, something appeared to add to the mass apart from the protons in the nucleus. One leading explanation was that the nucleus also contained electrons and additional protons - the protons still contributed to their mass, but the negatively charged electrons canceled their positive charge. 

    There would be four protons and two electrons in the nucleus in the helium example to yield a mass of 4 but only a charge of 2. Also, Rutherford put forward the idea that a particle with mass, but no charge, could exist. He called it a neutron and imagined it as an electron and a paired proton. For any of these ideas, there was no evidence. Chadwick kept the issue in his mind as he worked on other things. His eye was caught by experiments in Europe, particularly those of Frederic and Irene Joliot-Curie. To track particle radiation, they used a different method. Chadwick repeated their experiments but sought a neutral particle - one with the same mass as a proton, but with zero charge. He was successful in his experiments. He was able to determine that there was a neutron and that its mass was about 0.1 percent higher than that of the proton. In one of his first papers titled "Possible Neutron Existence," Chadwick published his findings with characteristic modesty. In 1935, he won the Nobel Prize for his discovery.

    These findings of Chadwick’s study were quickly accepted, and Werner Heisenberg showed that the neutron could not be a pair of proton-electron subatomic particles, but was a different type of unique particles - the third piece of the atom to be found. This new idea changed the image of the atom dramatically and accelerated discoveries in atomic physics. Soon, physicists found that the neutron made an ideal "bullet" to bomb other nuclei. It was not repelled by similarly charged particles, unlike charged particles, and could smash right into the nucleus. Neutron bombardment had been applied to fission of the uranium atom for dividing its nucleus and releasing the enormous amounts of energy predicted by the equation E= mc2 of Einstein.