Dalton’s atomic theory
Elements compound and mixture of Class 8
About DALTON’S ATOMIC THEORY
Proposed atomic theory of matter. The main points of Dalton’s atomic theory are as follows:
- Matter is made up of extremely small, indivisible particles called atoms.
- Atoms of same substance are identical in all respects, i.e., they possess same size, shape, mass, chemical properties, etc.
- Atoms of different substances are different in all respects, i.e., they possess different sizes, shapes, masses, chemical properties, etc.
- Atom is the smallest particle that takes part in chemical reactions.
- Atoms of different elements may combine with each other in a fixed, simple, whole number ratio to form compound atoms.
- Atoms can neither be created nor destroyed, i.e., atoms are indestructible.
Limitations of Dalton’s atomic theory:
The main failures of Dalton’s atomic theory are :
- It failed to explain how atoms of different elements differ from each other, i.e., it did not tell anything about structure of the atom.
- It could not explain how and why atoms of different elements combine with each other to form compound-atoms or molecules.
- It failed to explain the nature of forces that bind together different atoms in a molecule.
- It failed to explain Gay Lussac’s law of combining volumes.
- It did not make any distinction between ultimate particle of an element that takes part in reactions (atom) and ultimate particle that has independent existence (molecule).
Modern atomic theory:
The main modifications made in this theory are as follows :
- Atom is no longer considered to be indivisible. It has been found that an atom has a complex structure and is composed of sub-atomic particles such as electrons, protons and neutrons.
- Atoms of the same element may not be similar in all respects. Atoms of same element may possess different relative masses. For example, we have two different types of atoms of chlorine, with relative masses 35 a.m.u. and 37 a.m.u. respectively. Such atoms of the same element, which possess different masses, are called isotopes.
- Atoms of different may be similar in one or more respects. There are certain atoms of different elements, which possess same relative masses. For example, relative mass of calcium and argon is same (40 a.m.u.). Such atoms of different elements, which possess same mass, are called isobars.
- Atom is the smallest unit, which takes part in chemical reactions. Though atom is composed of sub-atomic particles, yet it is the smallest particle, which takes part in chemical reactions.
The following table gives a comparative study of solids, liquids and gases based on the kinetic molecular theory of matter.
|
Property |
Solids |
Liquids |
Gases |
|
Packing of molecules |
The molecules are closely packed and therefore solids have a definite shape, definite volume, high density and negligible compressibility |
The molecules are loosely packed hence liquids do not have definite shape and take the shape of the container. Density of liquids is slightly lesser than solids and therefore they are slightly more compressible than solids |
The molecules are very loosely packed and hence gases have no definite shape, no definite volume, very low density and very high compressibility |
|
Intermolecular force of attraction |
The molecules have very strong intermolecular force of attraction due to which they are rigid, and expand very less on heating |
The molecules have moderate intermolecular force of attraction which is lesser than solids. These can thus flow and expand more than solids when heated |
The molecules have negligible intermolecular force of attraction and can thus flow and expand much more than solids and liquids when heated |
|
Kinetic energy |
The molecules possess very low kinetic energy and hence do not diffuse. |
The molecules possess higher kinetic energy than in case of solids and can diffuse in certain liquids |
The molecules possess very high kinetic energy and diffuse spontaneously and rapidly |
On the basis of the above parameters, it can be concluded that the arrangement of molecules is the most important factor which determines the state in which the matter exists. That means, the change in molecular arrangement in a particular substance, in turn, depends upon the external conditions of temperature and pressure. Hence the change in molecular arrangement can be brought about by changing the external conditions of temperatures and pressure.
Atomicity of elementary gases: Avogadro’s law is quite helpful to determine the atomicity of elementary gases.Atomicity of an elementary substance is defined as the number of atoms in a molecule of the element.
Average Atomic Mass : There are many cases where different atoms of the same element possess different relative masses. Such atoms of the same element which have different relative masses are called isotopes. In such cases, atomic mass of the element is average of relative masses of different isotopes of the element. For example, chlorine contains two types of atoms having relative masses 35 u and 37 u. The relative abundance of these isotopes in nature is in the ratio 3 : 1. Thus, atomic mass of chlorine is the average of these different relative masses as described below :
Atomic mass of chlorine = 35.5 u.
Thus, the atomic mass of an element may be defined as the average relative mass of an atom of the element as compared with the mass of an atom of carbon (C-12) taken as 12 u (or 12 amu).
Atomic weights of various elements are given in the following table:
Element Symbol Atomic Number Atomic weight (u)
Actinium Ac 89 (227)
Aluminium Al 13 26.98154
Americium Am 95 (243)
Antimony Sb 51 121.75
Argon Ar 18 39.948
Arsenic As 33 74.9216
Astatine At 85 (210)
Barium Ba 56 137.33
Berkelium Bk 97 (249)
Beryllium Be 4 9.01218
Bismuth Bi 83 208.9804
Boron B 5 10.81
Bohrium Bh 107 (265)
Bromine Br 35 79.904
Cadmium Cd 48 112.41
Calcium Ca 20 40.08
Californium Cf 98 (251)
Carbon C 6 12.011
Cerium Ce 58 140.21
Caesium Cs 55 132.9054
Chlorine Cl 17 35.453
Chromium Cr 24 51.996
Cobalt Co 27 58.9332
Copper Cu 29 63.546
Curium Cm 96 (247)
Dysprosium Dy 66 162.50
Dubnium Db 105 (260)
Einsteinium Es 99 (254)
Erbium Er 68 167.26
Europium Eu 63 151.96
Fermium Fm 100 (253)
Fluorine F 9 18.998403
Francium Fr 87 (223)
Gadolinium Gd 64 157.25
Gallium Ga 31 69.72
Germanium Ge 32 72.59
Gold Au 79 196.9665
Hafnium Hf 72 178.49
Hassium Hs 108 (266)
Helium He 2 4.00260
Holmium Ho 67 164.9304
Hydrogen H 1 1.0079
Indium In 49 114.82
Iodine I 53 126.9045
Iridium Ir 77 192.22
Iron Fe 26 55.847
Krypton Kr 36 83.80
Lanthanum La 57 138.9055
Lawrencium Lr 103 (257)
Lead Pb 82 207.2
Lithium Li 3 6.941
Lutetium Lu 71 174.967
Molecular weight : The masses of molecules are also determined relative to the mass of carbon atom (C-12) taken as 12 u. These relative masses of molecules are called molecular masses. Molecular mass of a substance (element or compound) may be defined as the average relative mass of a molecule of the substance as compared with mass of an atom of carbon (C-12) taken
Gram molecular weight (GMW): It may be defined as that much quantity of the substance (element or compound) whose mass in gram is numerically equal to its molecular mass. Gram molecular mass is also called one gram-molecule of the substance. For example, molecular mass of ammonia is 17 u.
