NCERT Solutions For Class 9 Science Chapter 4 Structure of the Atom

Structure of the Atom – Important Questions with Answers

1. Who discovered the electron and how?
Answer: J.J. Thomson discovered the electron using cathode ray experiments.

2. What are the three subatomic particles in an atom?
Answer: Electron, proton, and neutron.

3. Name the scientist who discovered the nucleus of an atom.
Answer: Ernest Rutherford.

4. State any two observations of Rutherford’s alpha-particle scattering experiment.
Answer:

• Most alpha particles passed through the gold foil without deflection.

• A few were deflected at large angles, and very few rebounded.

5. What conclusion did Rutherford draw from his experiment?
Answer:

• Atom has a small, dense, positively charged nucleus.

• Most of the atom is empty space.

6. What are the drawbacks of Rutherford’s model?
Answer:

• It could not explain the stability of atoms.

• It did not explain the atomic spectra of elements.

7. What is Bohr’s model of the atom?
Answer: Bohr suggested that electrons revolve around the nucleus in fixed energy levels called orbits or shells.

8. What are energy levels or shells?
Answer: Fixed circular paths around the nucleus where electrons revolve without losing energy.

9. Who discovered the neutron?
Answer: James Chadwick in 1932.

10. What is the charge and mass of a neutron?
Answer: Neutron has no charge (neutral) and its mass is nearly equal to that of a proton.

11. What is the atomic number of an element?
Answer: The atomic number is the number of protons present in the nucleus of an atom.

12. What is the mass number of an atom?
Answer: The mass number is the sum of protons and neutrons in the nucleus.

13. What are isotopes? Give one example.
Answer: Isotopes are atoms of the same element having the same atomic number but different mass numbers.
Example: Hydrogen – Protium (¹H), Deuterium (²H), Tritium (³H)

14. What are isobars? Give one example.
Answer: Isobars are atoms of different elements having the same mass number but different atomic numbers.
Example: Argon (⁴⁰Ar) and Calcium (⁴⁰Ca)

15. Write the distribution of electrons in a chlorine atom (atomic number 17).
Answer: K shell – 2 electrons, L shell – 8 electrons, M shell – 7 electrons

16. What is the maximum number of electrons that can be accommodated in a shell?
Answer: The maximum number is given by the formula 2n², where n is the shell number.

17. What is the valency of an element?
Answer: Valency is the number of electrons an atom needs to gain, lose, or share to become stable.

18. What is the valency of the following elements:
a) Oxygen
b) Sodium
c) Carbon
Answer:
a) 2
b) 1
c) 4

19. What is an electron configuration?
Answer: It is the arrangement of electrons in different shells around the nucleus of an atom.

20. Write the electronic configuration of the following elements:
a) Hydrogen (1)
b) Carbon (6)
c) Neon (10)
Answer:
a) 1
b) 2, 4
c) 2, 8

21. What is the importance of neutrons in the nucleus?
Answer: Neutrons add mass to the atom and help in stabilizing the nucleus by reducing repulsion between protons.

22. Give one use of isotopes in the medical field.
Answer: Isotope of iodine (I-131) is used in the treatment of goitre and thyroid disorders.

23. How does Bohr’s model explain stability of atoms?
Answer: Electrons in fixed orbits do not radiate energy, hence the atom remains stable.

24. How do atoms of different elements differ from each other?
Answer: Atoms of different elements have different atomic numbers (i.e., different numbers of protons).

25. What is meant by the nucleus of an atom?
Answer: The central, dense, and positively charged part of an atom containing protons and neutrons.