John Dalton’s Atomic Theory | Postulates, Drawbacks

In 1808, An English scientist John Dalton proposed the first scientific theory of matter. This theory is also known as Dalton’s atomic theory of matter. This atomic theory is mainly based on the laws of chemical combination.

The main postulates (or assumptions) of Dalton’s atomic theory of matter are as follows:

(1) All the matter consists of very small (or tiny) particles called atoms, which are indivisible. An atom is the smallest particle of an element that takes part in a chemical reaction.

(2) All atoms of an element are identical to each other. They have the same mass, shape, size, and chemical properties. For example, all atoms of Cu are identical in all respects, such as mass, shape, size, and chemical properties. Similarly, all atoms of Zn are identical in all respects, such as mass, shape, size, and chemical properties.

(3) Atoms of different elements are different in mass, size, and chemical properties. For example, all atoms of Cu are different from atoms of Zn in every respect of their mass, shape, size, and chemical properties.

(4) Atoms can neither be created nor destroyed or transformed into atoms of other elements in the chemical reactions.

(5) Chemical compounds are formed when atoms of different elements combine with each other in a fixed whole number ratios such as 1:1, 1:2, 2:3, and so on. For example, sodium combines with chlorine to form sodium chloride (NaCl) in a fixed whole number ratio 1:1. Similarly, calcium combines with chlorine to form CaCl2 in a fixed ratio 1:2.

(6) Atoms of two elements may combine in different proportions to form more than one compound. For instance, carbon combines with oxygen to form carbon monoxide in 1:1 proportion and carbon dioxide in 1:2 proportion. Similarly, iron forms several compounds in different ratio, such as FeO (1:1), Fe2O3 (2:3), and Fe3O4 (3:4).

(7) A chemical compound always has the same (or fixed) relative numbers and kind of atoms.

(8) When atoms of distinct elements take part in chemical reactions, they involve only reorganisation (i.e. rearrangement) of atoms.

John Dalton proposed his atomic theory based on the laws of chemical combination. For example, the postulate of Dalton’s atomic theory that “atoms can neither be created nor destroyed nor transformed into atoms of another element in chemical reactions” is the result of law of conservation of mass proposed by Lavoisier.

Another postulate of Dalton’s atomic theory is that “an element consists of atoms having fixed mass, and relative number and kind of atoms in a given chemical compound is fixed” came from the law of definite proportions proposed by Proust.

Merits of Dalton’s Atomic Theory

There are two main merits of Dalton’s atomic theory. They are as:

(a) Dalton’s atomic theory successfully explained the law of conservation of mass and some other laws of chemical combination.

(b) Atoms of elements take part in the chemical reaction is correct and accepted in modern atomic theory with experimental evidence till today.

Drawbacks of Dalton’s Atomic Theory of Matter

Dalton’s atomic theory of matter suffered from the following drawbacks or limitations. They are as

(a) One of the major drawbacks of Dalton’s atomic theory of matter is that atoms had been thought to be indivisible (which cannot be further divided). Today we know that an atom can be further divided into smaller particles known as electrons, protons, and neutrons.

(b) According to Dalton’s atomic theory, all atoms of an element have exactly the same mass. But, it is seen that atoms of same element can have slightly different masses called isotopes. They are called isotopes. For example, 17Cl35 and 17Cl37 , both are isotopes. Other examples of isotopes are:

  • 8O16, 8O17 , 8O18
  • 1H1 , 1H2 , 1H3

(c) According to Dalton’s atomic theory of matter, atoms of different elements have different masses. But, it is seen that even atoms of different elements can have the same mass. For example, 20Ca40 and 18Ar40 , calcium and argon found to have the same mass number.

(d) Atoms combine together in a simple ratio to form compound but many compounds does not exist into simple ratio. Such compounds are called non-stoichiometric compounds but these compounds exist rarely. Example of such compounds are Fe0.93O, Fe0.95O, Ni0.98O.

(e) Dalton successfully explained the laws of chemical combination by mass but didn’t explain the Gay-Lussac’s law of gaseous volumes.

(f) He failed to explain why atoms of distinct elements are different in respect of mass, shape, size, valency, etc.

(g) He also failed to give any explanation as to why do atoms of same or different elements combine to form molecules.

(h) He also failed to provide any explanation about the nature of binding force between atoms and molecules, which is responsible for the existence of matter in three different states: solid, liquid, and gas.

Because of these above drawbacks or limitations, all the postulates of Dalton’s atomic theory of matter have been modified based on the modern atomic theory. Let’s understand them one by one.

Modified Dalton’s Atomic Theory (Modern Atomic Theory)

Following points of Dalton’s atomic theory has been modified on the account of researches made by scientists in the field of radioactivity, isotopes, isobars, and structure of atom. They are:

(a) Atom is divisible:

Atom is not the smallest particle of matter. It can be further divided into subatomic particles, such as electrons, protons, and neutrons.

(b) Atoms of the identical element are not alike:

Due to the discovery of isotopes, it has been found that atoms of an element may have different mass. Atoms of identical element which have the same atomic number but different mass number is called isotopes. For example, the element oxygen has three isotopes with atomic masses of 16amu, 17amu, and 18 amu.

(c) Atoms of different elements may not be different in all respect:

Due to discovery of isobars, atoms of different elements may also have the same atomic mass. Atoms of different elements having the same atomic mass but different atomic numbers are called isobars. For example, atoms of calcium and argon have the same mass number 40 but show different chemical properties.

(d) Atoms may not necessarily combine in simple ratios:

Atoms of different elements may combine with one another in a fixed ratio but may not always be simple. For example, the ratio of atoms in the compound cane sugar (C12H22O11) is 12:22:11 which is fixed but no simple.

(e) Atoms cannot be supposed as indestructible:

Dalton’s atomic theory of matter said that atoms cannot be created or destroyed or transformed into atoms of other elements. But, atoms of one element can transform into atoms of another element in the nuclear reactions. For instance, atoms of hydrogen element are continually fusing to form helium in the sun.

Brief Biography of an English Scientist John Dalton

An English scientist, John Dalton was born in 1766 in a poor weaver’s family at a village in England. He received his early education at the village school with the help of his father. At the age of 12, he started teaching in the village.

In 1793, he left for Manchester to teach chemistry, mathematics, and physics in a college students. John Dalton explained the theory of colour blindness in 1794. In 1808, he presented his first atomic theory to describe the properties of matter.

After presenting atomic theory of matter, he established a foundation for the modern chemistry. He was the first person to calculate the masses of atoms of various elements. He died in 1844.

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