Atomic hypothesis: Keeping in view various laws of chemical combinations, a theoretical proof for the validity of different laws was given by John Dalton in the form of hypothesis called Dalton's atomic hypothesis. Postulates of Dalton's hypothesis is as follows,
(i) Each element is composed of extremely small particles called atoms which can take part in chemical combination.
(ii) All atoms of a given element are identical i.e., atoms of a particular element are all alike but differ from atoms of other element.
(iii) Atoms of different elements possess different properties (including different masses).
(iv) Atoms are indestructible i.e., atoms are neither created nor destroyed in chemical reactions.
(v) Atoms of elements take part to form molecules i.e., compounds are formed when atoms of more than one element combine.
(vi) In a given compound, the relative number and kinds of atoms are constant.
Modern atomic hypothesis: The main modifications made in Dalton’s hypothesis as a result of new discoveries about atoms are,
(i) Atom is no longer considered to be indivisible.
(ii) Atoms of the same element may have different atomic weights. e.g., isotopes of oxygen O16, O17 and O18.
(iii) Atoms of different element may have same atomic weights. e.g., isobars Ca40 Ar40.
(iv) Atom is no longer indestructible. In many nuclear reactions, a certain mass of the nucleus is converted into energy in the form of α,ß and y rays.
(v) Atoms may not always combine in simple whole number ratios. e.g., in sucrose (C12H22O11), the elements carbon, hydrogen and oxygen are present in the ratio of 12 : 22 : 11 and the ratio is not a simple whole number ratio.
Berzelius hypothesis: “Equal volumes of all gases contain equal number of atoms under same conditions of temperature and pressure”. When applied to law of combining volumes, this hypothesis predicts that atoms are divisible and hence it is contrary to Dalton's hypothesis.