An atom consists of three subatomic particles which are – Protons, Neutrons and Electrons. Protons and neutrons are found in the nucleus of the atom while electrons are found in the orbits around the nucleus. In this article, we will discuss discovery of protons in detail but before that you need to know what protons and their fundamental properties are.
Protons are positively charged fundamental subatomic particles of atom. Its symbol is p or p+. As we know that a hydrogen atom consists of one proton and one electron, so on removing the electron from the hydrogen atom a proton is produced. That is why proton is also denoted as H+. It possesses +1e (or 1.60 10-19 coulomb) positive electric charge. Thus, protons have equal and opposite charge of electrons.
The word Proton is a Greek word which means ‘First’. It was 1st used by Ernest Rutherford in 1920. The subatomic particles protons and neutrons are together known as nucleons. Nucleons form the nucleus of an atom.
The mass of the proton is 1.67 10-24gram or 1.67 10-27kg. As we know the mass of an electron is 9.1 10-28 so the mass of a proton is 1836 times the mass of an electron. Although the mass of a proton is almost equal to the mass of a neutron.
Proton was found at the early stages of the investigations of atomic structure. English chemist William Prout gave Prout’s hypothesis in 1815 in which he proposed that all atoms are composed of hydrogen atoms. He called these hydrogen atoms ‘protyles’. Although it was disproved by various other discoveries and experiments later but set a fundamental base for discovery of various sub-atomic particles. Then in 1886, German physicist Eugen Goldstein carried out a modified cathode ray tube experiment in which he used electric discharge in a modified discharge tube with perforated cathode. In this experiment he discovered positively charged rays which he named as ‘canal rays’. Canal rays are also known as anode rays.
Let us investigate the discovery of protons through detailed explanation of the Canal Ray Experiment performed by Goldstein.
Goldstein used almost the same discharge tube as the cathode ray discharge tube used for discovery of electrons. The point of difference was perforated cathode used in Goldstein’s canal ray experiment.
Apparatus - Canal ray experiment apparatus consists of a tube (known as anode rays discharge tube) which is made of glass containing two thin pieces of metal, called electrodes, sealed in it. Out of these two electrodes one electrode (or one metal piece) has many holes in it which acts as a cathode while another electrode acts as anode. Both the electrodes are connected to the high voltage generator. The electrical discharge through the gases could be observed only at very low pressures and at very high voltages. So, the tube was evacuated so that pressure of different gases could be adjusted. When sufficiently high voltage is applied across the electrodes, current start flowing through a stream of particles in the tube.
Procedure and observation – Electric current was passed through both the electrodes which creates an electric field in the tube and ionizes the gaseous atoms in the tube. At this point anode starts emitting positively charged rays which move towards the negatively charged cathode and passes the holes of the cathode and strike the glass walls of the tube at the end near the cathode. These rays produced a faint red glow. These rays were passing in the opposite direction of the cathode rays so it was concluded that the charge they carry must be opposite of cathode rays. As these rays were moving from anode to cathode so called anode rays as well. Due to the same reason the glass tube used in this experiment was called anode rays discharge tube.
The characteristics of positively charged particles of canal rays are listed below –
Unlike cathode rays, mass of positively charged particles depends upon the nature of gas present in the anode ray tube. These simply the positively charged gaseous ions.
The charge to mass ratio of the particles depends on the gas from which these originate.
Some of the positively charged particles carry a multiple of the fundamental unit of electric charge.
The behavior of these particles in the magnetic or electrical field is opposite to that observed for electron or cathode rays in cathode ray’s experiment.
Thus, Eugen Goldstein 1st observed the protons although it was named and identified in other nuclei by British physicist Ernest Rutherford in 1919.
Physical properties of protons are as follows –
Charge of a proton – Protons are positively charged particles. A proton has 1.60 10-19 coulomb positive charge which is equal and opposite to that of an electron.
Mass of a proton – Mass of one proton is equal to that of the mass of one hydrogen atom. As a hydrogen atom contains only one proton in its nucleus and mass of an atom depends on its nucleus. Protons can be produced by removing an electron from a hydrogen atom. The mass of a proton is 1836 times the mass of an electron. Mass of a proton is 1.676 × 10-27 kg or 1.676 × 10-24 g.
Number of protons in an atom possesses its atomic number. For example, hydrogen contains one proton, so its atomic number is 1 while helium contains two protons, so its atomic number is 2.
Protons are spin -1/2 fermions. This property of protons is used in Proton NMR.
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