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What was the need of modifying Dalton's billiard ball model of the atom? When Dalton presented his billiard ball model, atoms were considered to be the smallest particle of matter. But after the discovery of negatively charged particles in the atom, Thomson presented his plum pudding model of the atom. The finding of the cathode ray experiment showed that cathode rays are part of all kinds of elements which means they are present in every atom. Why does Thomson's plum pudding model of the atom have a sea of positively charged particles rather than discrete particles? Atoms were considered to be neutral even at that time. So, to balance the negative particles with the positive ones, Thomson proposed that the rest of the atom is positively charged.  Why did Rutherford expect that the alpha particles would pass through the thin gold foil? Rutherford wanted to confirm Thomson's model of the atom according to which electrons are present in the sea of positive charge. As the whol...

Question Variations How did James Chadwick discover neutrons? Who discovered neutrons and how? What leads to the discovery of neutrons? In 1932, an English physicist James Chadwick found that when alpha particles were bombarded with beryllium, unknown radiation was detected which turned out to be particles and not electromagnetic radiation. He experimented further to find that electric and magnetic fields didn't impact these particles which showed that these particles were neutral.   Chadwick measured the mass of the neutron by bombarding beryllium with alpha particles. Then the proposed neutrons were directed towards the paraffin wax. Paraffin wax contains a good amount of hydrogen atoms which means they are rich in protons. So when the proposed neutrons would hit the paraffin wax, they should kick out these protons. So, by using a detector, the number of ejected protons could be calculated.  Then, Chadwick applied the law of conservation of energy and law of conservatio...