Page 1 - 3. LESSON NOTES-(THOMSON'SMODEL OF AN ATOM AND ITS DRAWBACKS)TS DRAWBACKS)
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SAI International School
                                                   Lesson Notes

                                                Subject-Chemistry
                                            Ch-Atoms and Molecules
                        Topic- Thomson’s model of an atom and its drawbacks

               To be done in CW Copy

               Introduction: J.J. Thomson, who discovered the electron in 1897, proposed the plum
               pudding model of the atom in 1904 before the discovery of the atomic nucleus in order to
               include the electron in the atomic model.
               Postulates of Thomson’s atomic model













               Postulate 1: An atom consists of a positively charged sphere with electrons embedded in it
               Postulate 2: An atom as a whole is electrically neutral because the negative and positive
               charges are equal in magnitude
               Thomson atomic model is compared to watermelon. Where he considered:
               •       Watermelon seeds as negatively charged particles
               •       The red part of the watermelon as positively charged
               Limitations of Thomson’s atomic model

               •       It failed to explain the stability of an atom because his model of atom failed to
               explain how a positive charge holds the negatively charged electrons in an atom. Therefore,
               this theory also failed to account for the position of the nucleus in an atom
               •       Thomson’s model failed to explain the scattering of alpha particles by thin metal foils
               •       No experimental evidence in its support


























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