In this post, we will discuss about Dalton’s Atomic Theory.
Dalton based his theory on two laws: the law of conservation of mass and the law of constant composition.
The law of conservation of mass says that matter is not created or destroyed in a closed system. That means if we have a chemical reaction, the amount of each element must be the same in the starting materials and the products. We use the law of conservation of mass every time we balance equations.
The law of constant composition says that a pure compound will always have the same proportion of the same elements. For example, table salt, which has the molecular formula NaCl, contains the same proportions of the elements sodium and chlorine no matter how much salt you have or where the salt came from. If we were to combine some sodium metal and chlorine gas—which I wouldn’t recommend doing at home—we could make more table salt which will have the same composition.
Postulates of Dalton’s atomic theory:
The main postulates of Dalton’s Atomic Theory are as follows:
(a) Matter is made up of extremely small, indivisible particles called atoms.
(b) Atom of same substance are identical in all respect i.e. they posses same size, shape, mass, chemical properties etc.
(c) Atom of different substances are different in all respect i.e. they posses different shape, size, mass and chemical properties etc.
(d) Atom is the smallest particle that takes part in chemical reactions.
(e) Atom of different elements may combine with each other in a fixed, simple, whole number ratio to form compound atoms.
(f) Atom 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 as follows:
(a) Atom was no more indivisible. It is made up of various sub-atomic particles like electrons, proton and neutron etc.
(b) It failed to explain how atoms of different elements differ from each other.
(c) It failed to explain how and why atoms of elements combine with each other to form compound atoms or molecules.
(d) It failed to explain the nature of forces that bind together different atoms in molecules.
(e) It failed to explain Gay Lussac’s law of combining volumes.
(f) It did not make any distinction between ultimate particle of an element that takes part in reaction (atoms) and the ultimate particle that has independent existence (molecules).
Summary
1. Dalton’s atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties.
2. Dalton based his theory on the law of conservation of mass and the law of constant composition.
3. The first part of his theory states that all matter is made of atoms, which are indivisible.
4. The second part of the theory says all atoms of a given element are identical in mass and properties.
5. The third part says compounds are combinations of two or more different types of atoms.
6. The fourth part of the theory states that a chemical reaction is a rearrangement of atoms.
7. Parts of the theory had to be modified based on the existence of subatomic particles and isotopes.
Modern Atomic Theory
The main feature of modern atomic theory is discussed below:
(a) Atom is no longer supposed to be indivisible. Atom has a complex structure and is composed of sub-atomic particles such as electrons protons and neutrons.
(b) Atom of the same element may not be similar in all respects e.g. isotopes.
(c) Atom of different elements may be similar in one or more respects e.g. isobars.
(d) Atom is the smallest unit which takes part in chemical reactions.
(e) The ratio in which atoms unite may be fixed and integral but may not be simple. e.g. In sugar molecules C12H22O11 the ratio of C, H and O atoms is 12:22:11 which is not simple.
(f) Atom of one element can be changed into atoms of other element for e.g. transmutation.
(g) Mass of atom can be changed in energy, i.e., E=MC2.
According to Einstein’s mass energy relation, mass and energy are inter-convertible. Thus atom is no longer indestructible.
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