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John Dalton was an English chemist, physicist, and meteorologist. He is best known for proposing the modern atomic theory and for his research into colour blindness, sometimes referred to as Daltonism in his honour.
The explanation for the law of conservation of mass and the law of definite proportions was provided by the English school teacher John Dalton in 1808. Dalton reasoned as follows –
( Ref. -McMurray & Fay, Chemistry, p. 40)
- Elements are made of tiny particles called atoms.
The concept of atoms explained why there were so many different elements.
- An element is characterized by the mass of its atom. Atoms of the same element have the same mass, but atoms of different elements have different masses.
As the law of definite proportions had shown that elements always combine in specific mass ratios, the distinguishing feature between atoms of different elements must be mass.
- Chemical combination of elements to make different substances occurs when atoms join together in small whole-number ratios.
Different samples of a pure compound (e.g. H2O – H : O :: 1 : 8) always contain the same proportion of elements by mass (the law of definite proportions) only if whole numbers of atoms combine. Fractional parts of atoms cannot be involved in chemical reactions.
- Chemical reactions only rearrange the way that atoms are combined; the atoms themselves are unchanged. Atoms can neither be created nor be destroyed in a chemical reaction.
For the law of conservation of mass to be valid, atoms must be chemically indestructible. If the same numbers and kinds of atoms are present in both reactants and products, then the masses of reactants and products must also be the same.
Dalton’s Atomic Theory
Dalton’s Atomic Theory can be stated as follows:
- Matter is discrete (discontinuous) and made up of very small particles called atoms.
- An atom is the smallest indivisible particle of an element which can take part in a chemical change.
- Atoms can neither be created nor be destroyed in a chemical reaction.
- Atoms are of various kinds. There are as many kinds of atoms as are elements.
- Atoms of the same element are identical in all respects, having the same size, shape and structure, properties and especially mass.
- Atoms of different elements have different properties, size, shape and structure, and different masses.
- Atoms of different elements can combine in a fixed ratio of small whole numbers to form compounds.
- Atoms of the same elements can combine in more than one ratio to form more than one compound.
Drawbacks of Dalton’s theory
In light of the current state of knowledge in the field of Chemistry, Dalton’s theory had a few drawbacks. According to Dalton’s postulates,
- An atom is the smallest indivisible particle of an element. However, it is now known that atoms can further be subdivided into elementary particles like electrons, protons, and neutrons.
- Atoms of the same element are identical in all respects, having the same size, shape and structure, and especially mass. Today, we know that atoms of the same element can have slightly different masses. Such atoms are called isotopes.
- Atoms of different elements have different properties and different masses. However, different elements do exist whose atoms have the same mass. Such atoms are called isobars.
Importance of Dalton’s theory
Notwithstanding these drawbacks, the importance of Dalton’s theory should not be underestimated. He displayed exceptional insight into the nature of matter. His ideas provided a framework that could be modified and expanded by later scientists. Thus, John Dalton is often considered to be the father of modern atomic theory.