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NMMS – Chemistry Structure of an Atom

  1. Main points
    Around 500 B C an Indian Philosopher Maharishi Kanad, postulated that if we go on dividing matter, we shall get smaller and smaller particles. He named these particles as paramanu. But his theory not accepted.
  2. 1808 – the English Chemist John Dalton suggested from his experimental work that each chemical element is made up of identical atoms, and that elements are different because they are each made of different atoms. This became known as Dalton’s Atomic Theory.
  3. In 1897 – British Chemist J. Thomson discovered that the particle which had negligible mass and negative charge. This particles are known as ‘electrons.‘
  4. In 1911, Rutherford discovered that the particle which has, positive charge and mass which is higher than electron, this particle is known as ‘protons.‘
  5. Rutherford proved that the entire positive charge of an atom concentrated at a particular part and this part is called nucleus.
  6. 1932, James Chadwick discovered another subatomic particle which had no charge and a mass nearly equal to that of  a proton. It was eventually named as neutron.
  7. The word ‘atom‘ originated from the Latin word ‘atomos‘ which means ‘that which cannot be divided.‘
  8. Atom which have three type of particles, positively charged particles, negatively charged particles, and also the particles which does not have any charge.
  9. The path in which electrons revolve around the nucleus of an atom is called ‘Orbit.‘


Objective Type Questions

  1. i) The formula for finding the maximum number of electrons that can be accommodated in a shell is ……..?
  2. a) 2n                 b) 1n2                   c) 3n2                             d) 2n2                  

Ans: d) 2n2  

  1. ii) Three Fourth (75%) of the entire matter of the universe is ……..?
  2. a) Oxygen      b) Nitrogen          c) Hydrogen               d) Carbon

Ans: c) Hydrogen  

iii) Number of protons in an atom is called its …………

  1. a) Atomic number         b) Atomic mass           c) Mass number        d) None of these Ans: a) Atomic number 
  2. iv) The nucleus of an atom has ………… charge.
  3. a) Chargeless                           b) Positive                     c) Negative                 d) Zero   

Ans: b) Positive

  1. v) In 1911, the scientist who discovered the centre of an atom
  2. a) Ruther Ford             b) J J Thomson             c) Dalton               d) Frederic Soddy   Ans: a) Ruther Ford


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