Page 1 - 3. LESSON NOTES-(BOHR'S MODEL OF AN ATOM)
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SAI International School


                                                Subject-Chemistry
                                               Ch-Structure of atom
                                        Topic- Bohr’s model of an atom

                                                   Lesson Notes

               Bohr’s Model of an atom
               Bohr came up with these postulates to overcome the objections raised against
               Rutherford’s model:

                     Only certain orbits known as discrete orbits of electrons, are allowed inside
                       the atoms

                     Electrons revolve around the nucleus in discrete orbits without emission of
                       radiant energy. Each orbit has a definite energy and is called an energy shell
                       or energy level.

                     An orbit or energy level is designated as K, L, M, N shells. When the electron
                       is in the lowest energy level, it is said to be in the ground state.
                     An electron emits or absorbs energy when it jumps from one orbit or energy
                       level to another.
                     When it jumps from a higher energy level to lower energy level, it emits
                       energy while it absorbs energy when it jumps from lower energy level to
                       higher energy level.























               Orbits
               Orbits are energy shells surrounding the nucleus in which electrons revolve.
               Discovery of neutrons: James Chadwick
               Hydrogen atom has one proton and one electron. Since the mass of protons is one
               unit and the electrons have negligible mass the mass of Hydrogen atom was found
               to be 1 unit. Helium atom which has 2 protons and 2 electrons its expected mass
               would have been 2 units but it was observed that its mass was 4 units. Thus,
               Chadwick concluded that the atoms consist of neutral particles whose masses are
               roughly equal to mass of proton and are present inside the nucleus.
               Properties of electrons, protons, and neutrons


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