Chemistry IIT & NEET Class 11th Class 12th Classes Competitive Exams Engineering

ATOMIC THEORIES

There are two atomic theories i.e. the Dalton’s atomic theory and the modified Dalton’s atomic theory which tells about the atoms. Dalton’s atomic theory has some limitations which are rectified(corrected) by Modified Dalton’s atomic theory.                                    

                      Dalton’s atomic theory           Modified Dalton’s atomic theory
John Dalton, a British school teacher published his theory about atoms in 1808.As a result of the researches made by various chemists and physicists, Dalton’s atomic theory has undergone radical changes but still Dalton’s name is present in modified Dalton’s atomic theory although he had not given it just because he was the first person who took the initiative for giving the atomic theory.
                          POSTULATES                               POSTULATES
All matter consists of indivisible particles called atoms.Atom is divisible i.e. atom is made up of electrons, protons and neutrons.
All atoms of a given element have identical properties including identical mass.Atoms of same element may have                        different atomic masses. Ex: Hydrogen atoms can have atomic mass 1, 2 or 3.  
Atoms of different elements differ in mass.  Atoms of different elements may have same atomic masses. Ex: Argon, potassium and calcium have atomic mass of 40.
Compounds are formed when atoms of different elements combine in a fixed simple whole number ratio. Ex: H20 has simple ratio of 2:1.The ratio in which different atoms combine with one another may be fixed and integral but may not always be simple. Ex: Cane Sugar C12H22O11 has complex ratio of 12:22:11.
Atoms are neither created nor destroyed in a chemical reaction.By carrying out nuclear reactions, atoms of an element may be changed into another. Ex: Uranium when gets hit by the neutrons gets changed into Plutonium and some mass of Uranium is converted into energy.

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