MCQ
Q.
Neutron was discovered by:
Correct Answer: C
Concept:
- Atoms are made up of subatomic particles - electrons protons and neutrons.
- In the center of an atom, a nucleus is present which contains protons and neutrons together known as nucleons.
- The mass of an atom is most concentrated at the center of the atom, i.e. the nucleus.
Electrons:
- Discovered by Sir J.J Thomson in the year 1897.
- Previously named as cathode rays as they were supposed to be emitted from the cathode.
- They are deflected towards the positive side.
- They have high kinetic energy.
- They are deflected by an electric and magnetic field.
- It has a negative charge of
\(charge=1.602\times10^{-19} coulombs \)
\(mass={1\over 1837}\times 9.107\times 10^{-28}g\)
Protons:
- Discovered by Goldstein.
- Depicted first as positive rays because it seemed to emerge from the anode.
- Mass of a proton is
\(1.670 \times 10^{- 24}g\)
\(Charge=1.602\times 10^{-19} coulombs\)
Explanation:
- Rutherford's discovery led to the discovery of nucleus.
- He found out that most of the mass of the nucleus is situated in its centre and contains the positively charged particles protons.
- The atom is otherwise empty and as electrons have negligible mass, the mass of an atom should be equal to the sum of mass of protons present in it.
- However, it was seen that in Helium, there are only two protons, so its mass should be twice the mass of each proton = \(2\times 1.008 a.m.u\).
- Experimental results revealed that it has a mass of 4.003a.m.u.
- Hence, it was concluded that the nucleus has particles containing no charge but mass almost equal to that of a proton.
- This was established by scientist James chadwick in 1932.
Additional Information
Rutherford's gold foil experiment:
- He discovered the atomic nucleus and gave us an atomic model.
- He directed alpha particles towards a metal sheet.
- He saw that most of the alpha particles, which are positively charged particles, are not getting deflected.
- Most of the alpha particles emerged undeflected.
- Therefore, he concluded that most of the mass of the nucleus is situated in its centre and contains the positively charged particles protons.
- Electrons revolve around the nucleus in certain orbits.
