Introduction An atom is the ultimate particle of all the elements of the universe, therefore the study of atomic structure is the most important topic. Properties of an element depend upon its atomic structure. The modern discoveries about constituents of an atom have revealed the existence of more than 35 constituents. Among them, three are important I.e., electron, proton and neutron. All these are constituents of an atom, thence they are called fundamental particles or sub-atomic particles.
Discovery of electron In nineteenth-century, Plucker, Crooks, etc scientists performed various experiments by passing an electric discharge through gases. At normal pressure, all gases are found to be non-conductor of electricity. If two electrodes of metal are placed in a long glass tube and about 10,000-volt electric potential is applied between the electrodes, there is no electric discharge. Now, if the tube is connected to a vacuum pump and pressure of the gas inside is reduced, then electricity is passed through it, a coloured glow is produced in the tube. On reducing the pressure further to about 10-4 atmosphere (0.01 mm) the wall of the tube facing the cathode is illuminated by fluorescence whose colour is greenish-yellow. The fluorescence is produced due to falling of a particular type of invisible rays on the phosphorescent material (ZnS) coated on the glass. These rays were named as cathode rays, by J.J. Thomson, being emerged from the cathode. These rays are made up of negatively charged particles. These particles are called electrons. The name electron was given by Stoney.
Properties of cathode rays (electrons) The main properties of electrons are as given below:
Charge to mass ratio of electron In 1897, J. J. Thomson determined charge to mass ratio of the electron using cathode ray tube. To apply an electrical and magnetic field perpendicular to each other and perpendicular to the path of electrons, also he used a self-made apparatus. According to Thomson under these conditions, the extent of deviation of electrons from their path is based on the following facts:
By carrying out correct measurement of the deflection from any one field Thomson determined the ratio of charge (e) and mass (me) which is e/me=1.758820 X 1011 C Kg-1
Charge on electrons: Millikan estimated the magnitude of negative charge present on an electron by oil drop method. Each electron has 1.602 X 10-19 coulomb charge which is minimum value of electric charge and its mass is 9.109 X 10-28g which is 1/1837th part of atomic mass of hydrogen as determined by Thomson on combining this value with e/me value. Discovery of electrons has proved that atoms are complex and have small constitutional units in them because charge present on an electron is the minium electric charge. Hence, charge present on an electron is considered as a unit negative charge.
Dicovery of proton Electron is an essential constituent of an atom and atom is electrically neutral hence, it is necessary that if an atom has negatively charged electrons, then positively charged particles must also be present for their neutralisation. Goldstein, in 1886 performed discharge tube experiment in which he took perforated cathode and gas at low pressure was filled inside the tube. He found that some rays emerged from the anode. These rays passed through the hole in the cathode and were positively charged. These rays travel from anode to cathode. These are also called positive rays or anode rays or canal rays. Study of positive rays obtained in discharge tube proves that atoms have positively charged particles in them but are not always same as that of the electron. Their mass is equal to the atomic mass of the gas taken in the tube. It is clear that positive rays are obtained from the gas present between the electrodes and they are formed when negatively charged electrons are ejected from the atoms of the gas, i.e., it is composed of positively charged particles. Study of rays obtained from various gases shows that ratio (e/m) of charge and mass of hydrogen gas is maximum. This concludes that if positive rays obtained from all gases have the same charge (e), then mass (m) of positively charged particle obtained from positive rays for hydrogen should be minimum. It is true and this statement has been proved by various types of studies. Therefore, it can be said that the positively charged particle obtained from hydrogen is a fundamental particle of matter. The positive charge of this particle is equal in magnitude to the negative charge of an electron, that is, 1.602 X 10-19 coulomb but the nature of charge is opposite. Its radius is 10-18 cm. Its mass is 1.672X10-24 g and is 1836 times heavier than an electron. In the atomic mass unit, it is 1.0073 a.m.u. which is approximately equal to the atomic mass of hydrogen. In normal calculations, this mass is as one a.m.u. An atom of hydrogen, which is the smallest atom, is made up of one proton and one electron. As the weight of an electron is very less, hence it may be considered negligible. This lightest positively particle was called proton by Rutherford.
Discovery of neutron The neutral character of an atom hindered further discovery after the discovery of proton and electron. Chadwick after a long time, in 1932 discovered the neutron. On bombarding beryllium foil by alpha particles, Chadwick found the production of new neutral particle. This neutral particle was named as the neutron. It is slightly heavier than proton, i,e, 1.675X10-24 g or 1.0087 a.m.u. In general calculations, its mass is considered to be unity. Neutron is also considered the fundamental particle of an atom.
Things to remember So, it is clear that the atom is not the smallest particle. The atom is made of three fundamental particles. The number of electrons or protons is called the atomic number whereas the sum of protons and neutrons is called atomic mass.
Hakimuddin Kuwakhedawala is based in India with over 15 years experience as a teacher. Teaching and writing are his passion. Most of the articles of Hakimuddin are related to education, exam, environmental problems, etc.
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Very exhaustive and elaborate lecture material provided by the author. Students are always curious about the small particles that constitute matter. Interestingly we see the matter but our eyes have a limit to resolve things and we cannot see below a particular size and these particles are much much below that limit. So, we can see the bulk material but cannot visualise the atoms or its constituents as they are too little to see with our naked eyes. Science has indirect methods and schemes to find out the nature and characteristics of these particles which are the basic building blocks of any matter.