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SAI International School
                                                  Subject-Chemistry

                                             Ch-Atoms and Molecules
                              Topic-Dalton's atomic theory and its drawbacks
                                                    LESSON NOTE


               Atoms:  These  are  the  smallest  particle  of  matter  which  may  or  may  not  have  an
                    independent existence and take part in chemical reactions.


               Molecules: These are the smallest particle of matter which can exist independently.


               Dalton’s Atomic Theory

               1.  The matter is made up of indivisible particles known as atoms.
               2.  The properties of all the atoms of a given element are the same, including mass.
                    This  can  also be  stated as  all  the  atoms  of  an  element  have identical  mass  and
                    chemical  properties;  atoms  of  different  elements  have  different  masses  and
                    chemical properties.
               3.  Atoms of different elements combine in fixed ratios to form compounds.
               4.  Atoms  are  neither  created  nor  destroyed.  The  formation  of  new  products
                    (compounds)  results  from  the  rearrangement  of  existing  atoms  (reactants)  in  a
                    chemical reaction.
               5.   The relative number and kinds of atoms are constant in a given compound.

               Merits of Dalton's Atomic Theory


                   The atomic theory explains the laws of chemical combination (the Law of Constant
                    Composition and the Law of Multiple Proportions).
                   Dalton  was  the  first  person  to  recognize  a  workable  distinction  between  the
                    fundamental particle of an element (atom) and that of a compound (molecule).

               Drawbacks of Dalton’s atomic theory:


                   The indivisibility of an atom was proved wrong: an atom can be further subdivided
                    into protons, neutrons and electrons. However, an atom is the smallest particle that
                    takes part in chemical reactions.
                   According  to  Dalton,  the  atoms  of  same  element  are  similar  in  all  respects.
                    However, atoms of some elements vary in their masses and densities. These atoms
                    of different masses are called isotopes.
                 Dalton  also  claimed  that  atoms  of  different elements  are  different in  all  respects.
                    This has been proven wrong in certain cases: argon and calcium atoms each have
                    an atomic mass of 40 amu. These atoms are known as isobars.





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