Hypothesis of Dalton's atomic theory → The matter is made up of indivisible particles known as atoms. The properties of all the atoms of a given element are the same including mass. Atoms of different elements combine in fixed ratios to form compounds. Atoms are neither created nor destroyed. The formation of new products (compounds) results from the rearrangement of existing atoms (reactants). Atoms of an element are identical in mass, size and many other chemical or physical properties, but atoms of two-different elements differ in mass, size, and many other chemical or physical properties. Drawbacks of Dalton's atomic theory → It was proved that an atom is not indivisible. As an atom can be subdivided into electrons, protons and neutrons. According to Dalton Atomic Theory, atoms of an element are identical in mass, size and many other chemical or physical properties. But, practically we observe that atoms of several elements differ in their densities and masses. These atoms with the different masses are known as isotopes. According to Dalton Atomic Theory, atoms of two-different elements differ in mass, size and many other chemical or physical properties. However, this is not correct for all situations, e.g. Argon (Ar) and Calcium (Ca) atoms, each have an atomic mass of 40 amu. These atoms with similar atomic masses are isobars. According to Dalton Atomic Theory, when atoms of different elements (atoms of two or more elements) combine in simple whole number ratios, we get chemical compounds. But this is not true in case of complex organic compounds. Dalton's Atomic Theory fails to explain the existence of allotropes. This implies that the Dalton atomic theory fails to explain the differences in properties of charcoal, graphite, and diamond (allotropes of carbon). OR Limitations of Dalton's atomic theory → It failed to explain how atoms of different elements differ from each other i.e. It did not tell anything about the structure of the atom. It could not explain how and why atoms different elements combine with each other to form compound atoms or molecules. It failed to explain the nature of forces that bind together different atoms in a molecule. It failed to explain Gay Lussac's law of combining volumes. It did not make any distinction between ultimate particle of an element that takes part in reactions (atoms) and an ultimate particle that has independent existence (molecule)