Important Questions For All Chapters – Science Class 9
Short Questions
1. Who discovered the electron?
Answer: J.J. Thomson discovered the electron.
2. What are canal rays?
Answer: Canal rays are positively charged radiations discovered by E. Goldstein.
3. What is the charge and mass of a proton?
Answer: A proton has a charge of +1 and a mass of one unit.
4. Name the three sub-atomic particles of an atom.
Answer: Electrons, protons, and neutrons are the three sub-atomic particles of an atom.
5. Who discovered the nucleus of an atom?
Answer: Ernest Rutherford discovered the nucleus of an atom.
6. What is the atomic number?
Answer: The atomic number is the total number of protons present in the nucleus of an atom.
7. Who discovered the neutron?
Answer: J. Chadwick discovered the neutron in 1932.
8. What are isotopes?
Answer: Isotopes are atoms of the same element having the same atomic number but different mass numbers.
9. What are isobars?
Answer: Isobars are atoms of different elements having the same mass number but different atomic numbers.
10. What is the valency of hydrogen?
Answer: The valency of hydrogen is one.
Long Questions
1. Describe Thomson’s model of an atom.
Answer: Thomson proposed that an atom consists of a positively charged sphere with electrons embedded in it, like seeds in a watermelon. The positive and negative charges are equal in magnitude, so the atom is electrically neutral as a whole.
2. Explain Rutherford’s alpha-particle scattering experiment.
Answer: Rutherford made fast-moving alpha particles fall on a thin gold foil. He observed that most particles passed through, some were deflected at small angles, and very few rebounded. He concluded that most of the atom is empty, the positive charge is concentrated in a small nucleus, and electrons revolve around it.
3. What were the main features of Rutherford’s model of the atom?
Answer: (i) There is a positively charged nucleus at the center.
(ii) Nearly all the mass of the atom resides in the nucleus.
(iii) Electrons revolve around the nucleus in circular paths.
(iv) The size of the nucleus is very small compared to the atom.
4. What were the drawbacks of Rutherford’s model?
Answer: The model could not explain the stability of the atom. A revolving electron should lose energy and fall into the nucleus, but atoms are stable. Hence, Rutherford’s model was incomplete.
5. State Bohr’s postulates for the model of an atom.
Answer: (i) Only certain special orbits, called discrete orbits, are allowed inside the atom.
(ii) While revolving in these discrete orbits, electrons do not radiate energy.
6. How are electrons distributed in different shells?
Answer: The distribution of electrons follows the formula 2n², where n is the orbit number. The K shell holds 2 electrons, L shell 8, M shell 18, and N shell 32. Electrons are not filled in outer shells unless inner shells are filled.
7. Define valency and explain it with an example.
Answer: Valency is the combining capacity of an atom. For example, sodium has one electron in its outermost shell, so its valency is one, as it can lose one electron to achieve a stable configuration.
8. What is the difference between atomic number and mass number?
Answer: Atomic number is the number of protons in the nucleus, while mass number is the sum of protons and neutrons present in the nucleus.
9. Explain with examples what isotopes are.
Answer: Isotopes are atoms of the same element having the same atomic number but different mass numbers. Example: Hydrogen has three isotopes – protium (¹₁H), deuterium (²₁H), and tritium (³₁H).
10. What are isobars? Give one example.
Answer: Isobars are atoms of different elements that have the same mass number but different atomic numbers. Example: Calcium (⁴⁰₂₀Ca) and Argon (⁴⁰₁₈Ar) are isobars.
11. What is the significance of neutrons in an atom?
Answer: Neutrons have no charge and mass equal to that of a proton. They are present in the nucleus and contribute to the atomic mass without affecting the charge of the atom.
12. What is the importance of isotopes in daily life?
Answer: Some isotopes have special uses: uranium isotope is used as nuclear fuel, cobalt isotope in cancer treatment, and iodine isotope in treating goitre.
13. What are the rules for writing electronic configuration?
Answer: (i) Maximum electrons in a shell = 2n².
(ii) The outermost shell can have up to 8 electrons.
(iii) Inner shells must be filled before filling outer shells.
14. Explain the stability of noble gases using electronic configuration.
Answer: Noble gases have completely filled outermost shells (helium has 2, others have 8 electrons). This full outer shell makes them chemically inactive and stable.
15. What did Rutherford conclude about the structure of the atom from his experiment?
Answer: He concluded that the atom has a small, dense, positively charged nucleus where most of the mass resides, and electrons revolve around it in the empty space.
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