CBSE – 9th Standard Science


Class 9 Question Bank Chapter 4 – Structure of the Atom

Q1. What are Electrons?

Ans.  Atom is not a simple, indivisible particle but contained at least one sub-atomic particle called the electron.

Q2. Who discovered Canal rays?

Ans. E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays.

Q3. What are Canal rays?

Ans. Canal Rays were positively charged radiations.

Q4. What is proton?

Ans. The sub-atomic particle equal in magnitude but opposite in sign to that of the electron is called proton.

Q5. What is Thompson’s model of an Atom?

Ans. An atom consists of a positively charged sphere and the electrons are embedded in it.

Q6. What is Rutherford’s model of an Atom?

Ans. Most of the space inside the atom is empty.

Q7. What is Bohr’s model of an Atom?

Ans. According to this model only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

Q8. What are Neutrons?

Ans. J. Chadwick discovered another subatomic particle which had no charge and a mass.

Q9. What are Valence Electrons?

Ans. The electrons present in the outermost shell of an atom are known as the valence electrons.

Q10. What we know about Bohr-Bury scheme?

Ans. We know that the outermost shell of an atom can accommodate a maximum of 8 electrons.

Q11. What is valency?

Ans. An atom of each element has a definite combining capacity, called its Valency.

Q12. What do you mean by Atomic Number?

Ans. The atomic number is defined as the total number of protons present in the nucleus of an atom.

Q13. What do you mean by mass Number?

Ans. The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom.

Q14. What are Isotopes?

Ans. Elements which have the same atomic number but different mass numbers.

Q15. What are Isobars?

Ans. Atoms of different elements with different atomic numbers, but same mass number.


How will you find the valency of chlorine, sulphur and magnesium?

On the basis of Thomson’s model of an atom, explain how the atom

is neutral as a whole.

  1. On the basis of Rutherford’s model of an atom, which subatomic

particle is present in the nucleus of an atom?

  1. Draw a sketch of Bohr’s model of an atom with three shells.
  1. What do you think would be the observation if the á-particle

scattering experiment is carried out using a foil of a metal other than gold?