John Dalton presented the atomic theory which was the first to describe matter as a divisible particle and established that matter is in fact composed of further small particles called atoms.
- The idea of matter being made up of small particles is very old and almost dates back to the pre-historic civilizations of Greece and India.
- The term atom is in fact derived from the Greek word “ATOMOS” meaning indivisible and was coined by Greek philosophers Leucippus and Democritus.
- In the 18th century Laws of conservation of mass and law of definite proportions were coined by Lavoisier and Proust
- John Dalton expanded on their work and put forward the Atomic Law.
BASIS FOR ATOMIC THEORY:
John Dalton based his atomic theory on two laws; Law of conservation of mass and law of definite proportions. His theory in fact provided evidence for the validity of the laws of chemical combinations
POSTULATES OF ATOMIC THEORY:
Dalton’s atomic theory consists of five postulates which are as follows
All matter is made up of atoms.
Dalton postulated that all matter was composed of small indivisible particles called atoms. He went on to present the hypothesis that the law of conservation of mass and law of definite proportion could be explained using the concept of atoms. However, his idea of an atom was to be a hard, massy, solid, impenetrable particle.
- suppose we have a copper coin.
- Before the atomic theory this copper coin was termed as indivisible matter.
- However, Dalton through his theory implied that a copper coin in fact is composed of copper atoms
Atoms cannot be created nor destroyed
Dalton’s atomic theory postulated atoms as fundamental particles that are not further divisible. He also went on to state that atoms in a chemical reaction recombine to form new compounds but no new atom is created and no existing atom is destroyed during a chemical reaction.
Suppose we have the reaction of water
2H2 + O2 —————————————> 2H2O
- In the above reaction two molecules of hydrogen combine with one molecule of oxygen to form water.
- Now we know that a water molecule is composed of two hydrogen atoms and one oxygen atom.
- This goes on to show that the same hydrogen and oxygen atoms rearranged to form a water molecule.
- No new atom was created in this reaction nor was an existing atom destroyed.
Atoms of the same element are identical but differ from atoms of other elements.
Dalton postulated that atoms of the same element are identical in their mass and chemical properties. At the same time atoms of different elements are different in regard to their mass and chemical properties
- Two atoms of oxygen will have the same chemical properties however; an atom of oxygen will have chemical properties different from that of a carbon atom.
- This postulate is mostly true as the discovery of isotopes has altered the opinion on the identical masses which would be discussed later.
Atoms combine in a ratio of small whole numbers to form compounds
According to this postulate, Compounds are produced by the combination of atoms however; they do so in ratios of simple whole numbers
- Sodium, a highly reactive metal combines with chlorine, a toxic gas to form salt which we use in our food.
- A molecule of salt (NaCl) consists of sodium and chloride in the ratio 1:1 which is in simple whole numbers.
- We can never have salt as Na0.5Cl0.5 because that would mean that half atom of sodium is interacting with half an atom of chloride which is not possible.
The relative number and kinds of atoms are always the same in a given compound.
The fifth postulate states that whatever may be the source, origin or quantity of a compound, it will always contain atoms of the same elements combined in the same whole number ratio.
- From the above diagram we can see that the compound of water will always contain 2 parts of hydrogen and one part of oxygen (ratio of hydrogen to oxygen is 2:1).
- This atomic composition and the atomic ratio of water will always be the same irrespective of the source, origin and quantity of water.
Drawbacks of Dalton’s Atomic Theory:
Dalton’s atomic theory has laid the basics for countless advancements and research in chemistry. However, due to the advancement of experimentation and technology, some drawbacks have been pointed out in this theory which are as under.
- The atom is no longer considered as an indivisible particle. We now know that an atom is composed of electrons, protons and neutrons. However, it is still the smallest particle that takes part in a chemical reaction.
- We now know that atoms of the same element can have different masses in the form of isotopes for example chlorine has two isotopes with mass numbers 35 and 37.
- The law postulated that atoms of different elements are different in all respects. With the discovery of isobars, we now know that atoms of different elements can possess similarities in some properties.
- According to Dalton, atoms of different elements combine in the ratio of simple whole numbers. This is not true for complex organic compounds i.e. sugar (C12H22O11).
- This theory falls short in providing an explanation for the occurrence of allotropes (existence of an element in more than one physical form).
- Despite all the shortcomings the atomic theory might have, it has been a revolutionary discovery in the history of chemistry.
- Dalton’s atomic theory has laid the foundation of various other discoveries in the field of chemistry.
- It has provided stern evidence regarding the validity of the laws of chemical combination.
- This was the first theory that provided a workable distinction between the fundamental particle of a compound (molecule) and that of an element (atom).
- Dalton’s atomic theory is still widely accepted to this day.