DISCOVERY OF CHARGED PARTICLES IN MATTER – Electron, Proton and Neutron
Discovery of Electron –
During 1890s’ many scientists performed experiments using cathode ray tubes. A cathode ray tube or discharge tube is made of glass from which most of the air has been removed. Such a cathode ray tube can be seen in figure. There are two metal electrodes- the negatively charged electrode is called cathode whereas the positively charged electrode is called anode.
An English physicist J.J. Thomson studied electric discharge through a cathode ray tube. When high voltage was applied across the electrodes, the cathode emitted a stream (cathode ray) of negatively charged particles, called electrons. Since the electrons were released from the cathode irrespective of the metal used for it or irrespective of the gas filled in the cathode ray tube, Thomson concluded that all atoms must contain electrons. When high electrical voltage is applied, the electrical energy pushes out some of the electrons from the atoms of the gas.
“ An electron (e–) is an extremely light, negatively charged sub-atomic particle.”
Characteristics of an Electron –
- Symbol – e– (e for electon ; minus sign for negative charge)
- Location – outside the nucleus
- Mass – The mass of an electron is about 1/1840 of the mass of a hydrogen atom. Hydrogen atom has lowest mass and is considered as 1u. Thus, the relative mass of an electron is (1/1840) u. The absolute mass of an electron is, however, 9.1 x 10-28 g. As the mass of an electron is extremely small, it is therefore considered nearly mass less / negligible in comparison with a proton or a neutron. Thus, electron mass does not include while calculating the atomic mass of an element or the mass number of an atom.
- Charge – The absolute charge on an electron is 1.6 x 10-19coulomb with negative charge. It has been found to be the lowest negative charge carried by any particle and therefore, is considered as the unit of negative charge. It means, the relative charge of an electron is, -1.
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Discovery of Proton –
E. Goldstein (1900) discovered protons in Anode Ray experiments. Atoms are electrically neutral. Hence after the discovery of the negatively charged constituent (electron) of an atom, attempts were made to discover the positively charged counterpart of electrons. By using a discharge tube containing a perforated cathode, Goldstein (1886) found that some rays (canal rays or anode rays) given out by the anode passed through these holes in a direction opposite to that of the cathode rays. These are called the positive rays or canal rays. The mass and charge of the canal ray particles depend on the nature of gas taken in the discharge tube. It means, different gases give different types of canal rays which contain positively charged particles having different masses and charges.
When hydrogen gas was filled in the discharge tube the positive charge on the positive rays was equal to the negative charge on an electron, and the mass was very slightly less than the hydrogen atom (or nearly equal to H-atom). Hydrogen atom has lowest mass and is considered as 1u. It contains only one proton in the nucleus whereas atoms of all other elements contain more than one proton. It means, a proton is formed by the removal of an electron from a hydrogen atom. When high electrical voltage is applied, the electrical energy pushes out the electron from the hydrogen atom and form canal ray.
“ A proton (p+) is a positively charged sub-atomic particle.”
Characteristics of a proton –
- Symbol – p+(p for proton ; plus sign for positive charge)
- Location – inside the nucleus
- Mass – Hydrogen atom contains one proton and one electron. As the mass of an electron is extremely small, it is therefore considered that the mass of a proton is equal to the mass of a hydrogen atom i.e. 1u. Thus, the relative mass of a proton is 1u or 1840 times of an electron. The absolute mass of a proton is, however,
1.6 x 10-24g.
- Charge – The charge of a proton is equal and opposite to the charge of an electron. The absolute charge of a proton is 1.6 x 10-19coulomb with positive charge. It has been found to be the lowest positive charge carried by any particle and therefore, is considered as the unit of positive charge. It means, the relative charge of a proton is, +1.The formation of cathode rays show that atoms contain negatively charged particles (electrons) and the formation of anode rays show that atoms contain positively charged particles (protons).
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Discovery of Neutron –
After the discovery of electrons and protons, it was assumed that the mass of helium atom (contains two
protons) should be double than that of a hydrogen atom (contains only one proton). (The electron being very light weight particle as compared to that of a proton, its contribution to the atomic mass can be ignored). But, the actual ratio of helium and hydrogen masses is 4:1. Rutherford and others, thus, suggested that there must be one more type of subatomic particle present in the nucleus which may be neutral but must have mass. Later in 1932, James Chadwick showed the existence of this third type of sub-atomic particle. This was named as neutron. The neutron was found to have a mass nearly equal than that of a proton. Thus, if the helium atom contained 2 protons and 2 neutrons in the nucleus, its mass ratio to hydrogen as 4:1 could be explained.
Except hydrogen atom, atoms of all other elements contain all three fundamental particles – electrons, protons and neutron. Hydrogen atom does not contain neutron.
Characteristics of a neutron –
- Symbol – n
- Location – inside the nucleus
- Mass – The mass of a neutron is equal to the mass of a proton. It means, the relative mass of a proton is 1u. and the absolute mass is 1.6 x 10-24g.
- Charge – Neutron is electrically neutral i.e. no charge.
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