In accordance with Dalton’s atomic theory, atoms of distinct elements react with each other in the simple ratios. Gay-Lussac proposed that gases react with each other in the simple ratio of their volumes, provided all gases are at the same temperature and pressure.
Jacob Berzelius attempts to correlate Dalton’s atomic theory with Gay-Lussac law of gaseous volumes, and stated that equal volume of all gases at the same conditions of temperature and pressure contains the same number of atoms.
This hypothesis was later on found to be incorrect because it failed to explain the experimental results and contradicted the very basic postulates of Dalton’s atomic theory of matter, i.e., an atom is indivisible.
For example, Berzelius hypothesis could not explain the formation of hydrogen chloride from hydrogen and chlorine.
From the above figure, it is clear that for the formation of 1 compound atom of hydrogen chloride, 1/2 atom of hydrogen and 1/2 atom of chlorine are required. Here, each atom of hydrogen and chlorine has been divided, which is against Dalton’s atomic theory. Thus, Berzelius hypothesis was discarded.
Avogadro Hypothesis
In 1811, the Italian scientist, Amedeo Avogadro solved the above problem by proposing two types of particles. He said the whole matter comprises two types of particles. They are:
(1) Atom:
Atom is the smallest particle of an element that takes part in a chemical reaction but generally cannot exist free.
(2) Molecule:
Molecule is the smallest particle of a substance (element or compound) which exists independently and possesses all characteristic properties of the substance.
A molecule of an element is made up of like atoms while a molecule of a compound comprises a fixed number of atoms of two or more distinct elements. A molecule may be decomposed (or broken down) into its constituent atoms, but the atom is indivisible during a chemical change or reaction.
After making the above difference, Avogadro proposed a hypothesis known as Avogadro’s hypothesis, which stated as follows:
At the same conditions of temperature and pressure, equal volumes of all gases contain an equal number of molecules. On the basis of this law, the Avogadro hypothesis explained successfully the formation of hydrogen chloride.
Avogadro could successfully explain Gay-Lussac’s law with his hypothesis. He assumed that both hydrogen and chlorine are diatomic in nature. Thus, the hypothesis states that the when gases react with each other, molecules of reacting gases break up into constituent atoms during chemical reaction, which then combine to form new molecules called products.
Applications of Avogadro’s Hypothesis
(1) Atomicity*: The hypothesis of Avogadro helps in determining the atomicity of elements. Atomicity is the number of atoms present in one molecule of substance. Avogadro’s assumption that hydrogen, nitrogen, oxygen, chlorine, etc., are diatomic in nature. Noble gases are monoatomic, whereas ozone is triatomic in nature.
(2) Molecular formula: Avogadro’s hypothesis also helps in determining the molecular formulae of gases. Two volumes of ozone after decomposition give three volumes of oxygen at the same conditions of temperature and pressure.
Thus, the molecular formula of ozone is O3.
In this tutorial, you will have known about Avogadro’s hypothesis and its applications with example. Hope that you will have understood the basic points of the hypothesis.
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