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  • J. J. Thomson proposed that an atom was a solid sphere of positively charged material and the negatively charged particles electrons were embedded in it like the seeds in a watermelon.
  • Rutherford’s alpha scattering experiment led to the discovery of the atomic nucleus.
  • Rutherford proposed that electrons were orbiting in a manner in the same way as the planets are orbiting the sun. Hence this model is also called as planetary model of atom.
  • Bohr proposed that electrons are distributed in different shells with various energy levels.
  • The atomic number of an element is the number of protons present in the nucleus of an atom.
  • Atoms of the same element, rather elements having same atomic number (Z), but different mass number (A) (rather atomic mass) are called isotopes.
  • Atoms of different elements with different atomic number but with the same mass number are called isobars.

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