Class 9

9.2.4 Discovery of Neutrons and Nucleus

This is the fourth lecture from Chapter 2: ‘Atomic Structure’ of the new book for Class 9 Chemistry (Punjab Board – PCTB). It discusses the discovery of neutrons and nucleus. The lecture includes a multiple-choice quiz, short-answer questions, and detailed long-answer notes.

MCQs Based Quiz

9.2.4 Discovery of Neutrons and Nucleus

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1. The charge on an electron is:

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2. The charge on a proton is:

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3. The charge on a neutron is:

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4. The mass of a neutron is:

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5. The mass of a proton is:

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6. The mass of an electron is:

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7. In the nucleus of an atom, protons and neutrons are held together by:

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8. Almost all the mass of an atom is concentrated in its nucleus. What is the reason behind this?

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9. According to Rutherford an atom has ___ portions.

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10. Almost 99.9% of the mass of an atom reside in:

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11. Which pair of sub-atomic particles have almost the same mass?

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12. Which sub-atomic particle is electrically neutral?

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13. Which sub-atomic particle was discovered in 1932?

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14. Which of the following are sub-atomic particles

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The average score is 100%

Short Questions

How does the mass of a neutron compare to a proton?

They both have nearly the same mass:

  • Mass of Proton = 1.673 x 10-27 kg
  • Mass of Neutron = 1.675 x 10-27 kg

Are there any atoms made up of only two sub-atomic particles?

Yes, there is one such atom: protium ($\ce{^{1}_{1}H}$), which is the most common isotope of hydrogen. A protium atom consists of only one proton and one electron.

Why are electrons, protons, and neutrons called “fundamental particles”?

They are called fundamental particles because they are the basic building blocks of atoms. All atoms contain these particles, except protium, which lacks a neutron.

Why is almost all the mass of an atom concentrated in the nucleus?

Almost all the mass of an atom is concentrated in the nucleus because protons and neutrons, which are much more massive than electrons, are located there.

What force holds protons and neutrons together in the nucleus?

Protons and neutrons are held together in the nucleus by the strong nuclear force.

Descriptive Question

Write a note on the discovery of nucleus and neutrons.

Discovery of Nucleus:

In 1911, Ernest Rutherford performed his famous gold foil experiment, which led to the discovery of the atomic nucleus. In this experiment, special type of particles were bombarded at a very thin sheet of gold foil. Based on the results, Rutherford made the following conclusions:

  1. An atom consists of two main regions: the nuclear portion and the extra-nuclear portion.
  2. Nuclear portion, also called nucleus, is located at the center of the atom.
  3. Almost all the mass of the atom is concentrated in the nucleus.
  4. The extra-nuclear region surrounds the nucleus and contains electrons moving in specific paths or orbitals.

Discovery of Neutrons:

Neutrons were discovered in 1932. They are sub-atomic particles with no charge and a mass equal to that of a proton.

Current Concept of Nucleus:

After the discovery of neutrons, scientists explained that:

  1. The nucleus is made of protons and neutrons.
  2. These particles are held together by a strong nuclear force.
  3. Protons and neutrons are heavy, so most of the mass of an atom is present in the nucleus.