(b) The smallest particle of a matter that can exist in free state in nature, is known as a molecule.
(c) Some molecules are composed of homoatomic atom, e.g., H2, O2, N2, Cl2, O3 etc., while the molecules of compounds are made up of two or more heteroatomic atoms e.g., HCl, NaOH, HNO3, CaCO3, etc.
DALTON’S ATOMIC THEORY
The concepts put forward by John Dalton regarding the composition of matter are known as Dalton’s atomic theory. Its important points are as follows.
(a) Every matter is composed of very minute particles, called atoms that take part in chemical reactions.
(b) Atoms cannot be further subdivided.
(c) The atoms of different elements differ from each other in their properties and masses, while the atoms of the same element are identical in all respects.
(d) The atoms of different elements can combine in simple ratio to form compounds. The masses of combining elements represent the masses of combining atoms.
(e) Atom can neither be created nor destroyed.
MODERN CONCEPT
Many of the concepts of Dalton’s atomic theory cannot be explained. Therefore, foundation of modern
atomic theory was laid down by the end of nineteenth century. The modern theory is substantiated by the existence of isotopes, radioactive disintegration, etc. The important points of the modern atomic theory are as follows.
(a) Prof. Henri Bacquerel discovered the phenomenon of radioactivity and found that an atom is divisible.
(b) An atom is mainly composed of three fundamental particles, viz. electron, proton and neutron.
(c) Apart from the aforesaid three fundamental particles, many others have also been identified, viz. positron, meson, neutrino, antiproton, etc.
(d) Soddy discovered the existence of isotopes, which were atom of the same element having different masses.
For example, protium, deuterium and tritium are atoms of hydrogen having atomic masses 1, 2 and 3 a.m.u. respectively.
(e) Atoms having same mass may have different atomic numbers. These are known as isobars. For example, 40/18 Ar ,and 40/20Ca
(f) Atoms of elements combines to form molecules.
(g) It is not necessary that the atoms should combine in simple ratio for the formation of compounds. The atoms in non-stoichiometric compounds are not present in simple ratio. For example, in ferrous sulphide crystals, iron and sulphur atoms are present in the ratio of 0.86 : 1.00.
atomic theory was laid down by the end of nineteenth century. The modern theory is substantiated by the existence of isotopes, radioactive disintegration, etc. The important points of the modern atomic theory are as follows.
(a) Prof. Henri Bacquerel discovered the phenomenon of radioactivity and found that an atom is divisible.
(b) An atom is mainly composed of three fundamental particles, viz. electron, proton and neutron.
(c) Apart from the aforesaid three fundamental particles, many others have also been identified, viz. positron, meson, neutrino, antiproton, etc.
(d) Soddy discovered the existence of isotopes, which were atom of the same element having different masses.
For example, protium, deuterium and tritium are atoms of hydrogen having atomic masses 1, 2 and 3 a.m.u. respectively.
(e) Atoms having same mass may have different atomic numbers. These are known as isobars. For example, 40/18 Ar ,and 40/20Ca
(f) Atoms of elements combines to form molecules.
(g) It is not necessary that the atoms should combine in simple ratio for the formation of compounds. The atoms in non-stoichiometric compounds are not present in simple ratio. For example, in ferrous sulphide crystals, iron and sulphur atoms are present in the ratio of 0.86 : 1.00.
(h) Atoms participate in chemical reactions.
Dry gases are normally bad conductors of electricity. But under low pressure, i.e., 0.1 mm of mercury or
lower, electric current can pass through the gases. Julius Plucker in 1859 found that a type of rays, called cathode rays, emit from the cathode when electricity is passed through a discharge tube. William Crookes (1879), J.J.Thomson and many other scientists studied the properties of cathode rays and came to the conclusion that the cathode rays of same properties are obtained using any gas or any cathode material
The salient features of cathode ray are as follows.
(a) Cathode rays travel in a straight line. This indicates that the formation of a shadow when an opaque object is placed in its path.
(b) If a light metal pinwheel is placed in the path of cathode rays, the wheel starts revolving. This proves that is cathode rays consist of tiny particles having momentum.
(c) Cathode rays get deviated in electrical and magnetic fields. This proves that they are composed of charged particles. Their derivation towards anode indicates their negatively charged nature. The direction of their deviation in magnetic field depends on pole of the magnet which has been placed near the cathode ray tube.
(d) Cathode rays produce green fluorescence on the walls of the glass tube.
(e) Cathode rays produce incandescence in at thin metal foil.
(f) Cathode rays effect the photographic plate.
(g) Cathode rays ionize gases proving that they are charged.
(h) Cathode rays penetrates across a thin metal foil.
(i) Cathode rays produce X-rays when they hit a piece of tungsten or any other metal having high melting point
POSITIVE RAYS OR CANAL RAYS : DISCOVERY OF PROTON
Eugene Goldstein in 1886 found that a dim glow is visible behind the cathode when an electric discharge is passed through a perforated cathode in a discharge tube filled with a gas at low pressure. These new type of rays travel from anode to that cathode. Goldstein gave the name canal rays to these rays because these rays cross the canals of the cathode and reach the other side. W.Wein in 1897 proved through experiments that the canal rays consist of positively charged particles. J.J. Thomson gave the name positive rays to them because they are composed of positively charged particles.
DISCOVERY OF NEUTRON
Penetrating rays are emitted on bombarding α-particles on the elements like beryllium, boron and aluminium.James Chadwick in 1932 studied the nature of these radiation and came to the conclusion that these rays are composed of very tiny electro neutral particles. The mass of these particles is almost equal to that of the hydrogen atom. This particle is called neutron and is denoted by the symbol, 0n1
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