We all know that all matter in the universe is made up of microscopically small particles called atoms. The word atom comes from the Greek word άτομο, which means indivisible. Many ancient philosophers (like Kanada from India & Democritus from Greece) had a vague qualitative idea of the ultimate constituents of matter. Their views are not based on any scientific observation as we know them today. After almost two millennia the atom was placed on a solid foothold as a fundamental chemical object by John Dalton. And the hypothesis he gave is now known as Dalton’s atomic theory.
Dalton’s Atomic Theory Was Published in which year?
In the year 1803, a British chemist named John Dalton first propounded the modern theory of the atomic constitution of matter, based on a number of well-known laws of chemical combination.
Three Laws Of Chemical Combination Upon Which Dalton’s Atomic Theory Is Based On
The laws of the chemical combination include
1. The Law Of Conservation Of Mass
2. The Law Of Definite Proportions
3. The Law Of Multiple Proportions
A ) The Law Of Conservation Of Mass
A French scientist Antoine Lavoisier in 1789 first propounded the law of conservation of mass.
On the basis of many experiments, the Law Of Conservation Of Mass states that the total weight of all the components taking part in a chemical reaction is equal to the weight of all the products of the reaction.
For example, If we heat up a piece of pure tin in a closed vessel so that it combines chemically with the oxygen of the air contained in the vessel, then the total weight of the substances inside the vessel plus the weight of the vessel remained the same before and after the chemical combination.
B ) The Law Of Definite Proportions
The French scientist J.L. Proust in 1799 first propound the law of definite proportions.
The Law Of Definite Proportions states that when two chemical elements A and B combine to produce a chemical compound C [say], then the elements A and B are present in a definite proportion by weight in the compound C.
For example, in the chemical compound water, irrespective of the method of production of water, the two elements hydrogen and oxygen are always present in the same ratio of 1.008:8 by weight.
C ) The Law Of Multiple Proportions
John Dalton himself in 1803 discovered the Law Of Multiple Proportions.
The law of multiple proportions states that when two elements A and B combine chemically to produce two or more chemical compounds, then the ratios of the weights of any one of them [say B] which combine with a fixed weight of the other [sayA] to form the different compounds C1, C2…. are equal to the ratio of small integers.
For example, we know that the elements carbon and oxygen combine to produce the two chemical compounds carbon monoxide [CO] and carbon dioxide[CO2]. In the one mole of the former [ i.e., CO] 12 × 10-3kg of carbon is in chemical combination with 16 × 10-3 kg of oxygen. In the latter [i.e. CO2] 12 × 10-3 kg of carbon is in chemical combination with 32 × 10-3 kg of oxygen. Thus the weight of oxygen which is in chemical combination with 12 × 10-3 kg of carbon in the above two compounds are in the ratio of 16:32 or 1:2.
Postulates Of Dalton’s Atomic Theory
On the basis of the above-stated laws of chemical combination, Dalton proposed his atomic hypothesis as follows:-
1 ) All chemical elements are made up of extremely small particles known as atoms. The atom can not be further subdivided by any chemical process. They retain their identity during chemical reactions.
2 ) All atoms of the same element are identical, i.e., their weights and other properties are exactly the same. The atoms of different elements defer in their weight and other properties. Each element is characterised by the weight of its atom.
3 ) When different elements combine chemically, it is the atoms of these elements which combine together. This combination of the atoms takes place in a simple numerical ratio i.e. 1:1, 1:2, 2:1, 2:3.
With the help of the above stated three postulates, it is possible to explain the law of chemical combination.
Explaining The Law Of Conservation Of Mass With The Help Of Dalton’s Atomic Theory Postulates:
Since the weight and other properties of the atoms of different elements and their numbers taking part in a chemical reaction remain unchanged during the reaction and since the total weight of each element taking part in the reaction is equal to the combined weight of all the atoms present in it, we conclude the total of all the substances produced in the reaction.
This is the Law Of Conservation Of Mass.
Explaining The Law Of Definite Proportions With The Help Of Dalton’s Atomic Theory Postulates:
Since chemical compounds are produced by the combination of the atoms of two or more elements in simple numerical proportions and since all the atoms of the same elements have equal weight, hence the elements taking part in a chemical reaction always combine together in definite proportions by weight.
This is the Law Of Definite Proportions.
Explaining The Law Of Multiple Proportions With The Help Of Dalton’s Atomic Theory Postulates:
Now consider the production of two types of chemical compounds C1, C2 by the combination of the two elements A and B. Suppose that in each molecule of the first compound C1, a1 atoms of A are in chemical combination with b1 atoms of B. If WA and WB are the weights of the atoms of A and B respectively, then the weights of A and B which are present in chemical combination in each molecule of C1 are a1WA and b1WB respectively.
Similarly if in each molecule of the second compound C2, a2 atoms of A are in chemical combination with b2 atoms of B, then the respective weights of A and B in each molecule of C2 are a2WA and b2WB. Hence the weight of B which is in chemical combination with the unit weight of A in C1 is b1WB/a1WA. Likewise, the weight of B which is in chemical combination with the unit weight of A in C2 is b2WB/a2WA. So the ratio of the weight of B which are in chemical combination with unit weights of A in the two compounds C1 and C2 is
Since a1, b1, a2, and b2 are small integers, the ratio a2b1:a1b2 is equal to the ratio of two small integers.
This is the Law Of Multiple Proportions.
Application Of Dalton’s Atomic Theory Postulates
On the basis of the third postulates of Dalton’s atomic theory scientists were able to determine the relative masses of the atoms of a number of elements taking the atomic mass of Hydrogen to be 1.
Problem With Dalton’s Atomic Theory
Because of the inadequacy of the experimental data regarding the numbers of atoms of the molecule of the compound, his results were in many cases incorrect.
For example, John Dalton assumed the numbers of hydrogen and oxygen atoms in a molecule of water to be one each (So aH = 1 & aO=1) from a knowledge of the ratio r of the weights of hydrogen and oxygen present in water determined experimentally 1:8. He estimated the relative atomic mass of oxygen from the relation r = aHWH/aOWO = ⅛, Taking WH = 1 he found WO = 8, which as we know today is not correct.
It was found that certain experimental facts regarding the combination of gases could not be properly reconciled with Dalton’s Atomic Theory. In order to explain those facts, Avogadro proposed his famous Avogadro hypotheses which you can read here.
Drawbacks of Dalton’s Atomic Theory
It Does Not Account For Subatomic Particles: Dalton’s atomic theory stated that atoms were indivisible. However, the discovery of subatomic particles (such as protons, electrons, and neutrons) disproved this postulate.
It Does Not Account For Isotopes: As per Dalton’s atomic theory, all atoms of an element have identical masses and densities. However, different isotopes of elements have different atomic masses (For example, hydrogen, deuterium, and tritium).
It Does Not Account For Isobars: This theory states that the masses of the atoms of two different elements must differ. However, it is possible for two different elements to share the same mass number. Such atoms are called isobars (Example: 40Ar and 40Ca).
Elements Need Not Combine In Simple, Whole-Number Ratios To Form Compounds: Certain complex organic compounds do not feature simple ratios of constituent atoms. Example: sugar/sucrose (C11H22O11).
The Theory Does Not Account For Allotropes: Dalton’s atomic theory is unable to explain the differences in the properties of diamond and graphite, both of which contain only carbon.