Class 9th Science Structure of the Atom Important QAs

🔹 Very Short Answer Questions (1 mark)

Q1. Who discovered the electron?
Ans. J.J. Thomson.

Q2. Who discovered the proton?
Ans. E. Goldstein (discovered canal rays → protons identified).

Q3. Who discovered the neutron?
Ans. James Chadwick in 1932.

Q4. What is the charge on an electron?
Ans. –1 (negative).

Q5. Name the experiment conducted by Rutherford.
Ans. Alpha particle scattering experiment.

Q6. Which particle is present in the nucleus of hydrogen atom?
Ans. Only one proton.

Q7. What is the symbol of neutron?
Ans. n⁰.

Q8. Define nucleus.
Ans. The small, dense, positively charged central part of an atom containing protons and neutrons.

Q9. What is the charge on a proton?
Ans. +1 (positive).

Q10. Who proposed the planetary model of an atom?
Ans. Rutherford.

🔹 Short Answer Questions (2–3 marks)

Q11. State the drawbacks of Thomson’s model of an atom.
Ans.

1. Could not explain results of Rutherford’s experiment.
2. Did not explain stability of atom.

Q12. Write any two observations of Rutherford’s experiment.
Ans.

1. Most α-particles passed straight through the gold foil.
2. A few particles were deflected or rebounded.

Q13. Write any two conclusions of Rutherford’s experiment.
Ans.

1. Atom has a small dense positively charged nucleus.
2. Most of the space in an atom is empty.

Q14. State Bohr’s model of an atom.
Ans.

1. Electrons revolve in fixed orbits (shells).
2. Energy of each orbit is fixed.
3. Electrons do not radiate energy in these orbits.

Q15. Define atomic number.
Ans. The number of protons in an atom is called its atomic number (Z).

Q16. Define mass number.
Ans. The sum of protons and neutrons in an atom is called its mass number (A).

Q17. Give one example of isobar.
Ans. ⁴⁰Ca and ⁴⁰Ar.

Q18. Give one example of isotope.
Ans. ¹²C, ¹³C, ¹⁴C.

Q19. What are nucleons?
Ans. Protons and neutrons present in the nucleus are called nucleons.

Q20. Why is an atom electrically neutral?
Ans. Because the number of protons (+ charge) equals the number of electrons (– charge).

🔹 Long Answer Questions (4–5 marks)

Q21. Explain the limitations of Rutherford’s model.
Ans.

1. Failed to explain why electrons do not fall into nucleus despite attraction.
2. Could not explain stability of atom.
3. Could not explain arrangement of electrons in shells.

Q22. Differentiate between isotopes and isobars.

Q23. State main postulates of Bohr’s model of an atom.
Ans.

1. Electrons revolve in fixed shells.
2. Energy of each shell is fixed.
3. Electrons gain/lose energy only when they move between shells.

Q24. What are the merits of Rutherford’s atomic model?
Ans.

1. Discovered the nucleus.
2. Explained that atom is hollow and most of mass is concentrated in nucleus.
3. Proposed planetary model.

Q25. Why are isotopes of an element chemically similar?
Ans. Because chemical properties depend on atomic number (electrons), and isotopes have same atomic number.

Q26. What are the applications of isotopes?
Ans.

1. Cobalt-60 used in cancer treatment.
2. Iodine-131 used in treatment of goitre.
3. Uranium isotopes used in nuclear power.

Q27. Distinguish between electron, proton and neutron.

Q28. State two characteristics of cathode rays.
Ans.

1. Travel in straight lines.
2. Negatively charged particles (electrons).

Q29. State two characteristics of canal rays.
Ans.

1. Consist of positively charged particles (protons).
2. Travel in straight lines opposite to cathode rays.

Q30. What are shells or energy levels?
Ans. The fixed circular paths around the nucleus in which electrons revolve are called shells (K, L, M, N…).

🔹 Case Study / Application-Based (5–6 marks)

Q31. Case Study – Gold Foil Experiment
Alpha particles were bombarded on a thin gold foil.
(i) What observations were made?
(ii) What conclusions were drawn?
Ans.
(i) Most passed straight, few deflected, very few rebounded.
(ii) Atom mostly empty, nucleus is dense and positively charged, nucleus is very small.

Q32. Case Study – Isotopes of Hydrogen
Hydrogen has three isotopes: Protium, Deuterium and Tritium.
(i) Write their composition.
(ii) Which one is radioactive?
Ans.

• Protium: 1 proton, 0 neutron.
• Deuterium: 1 proton, 1 neutron.
• Tritium: 1 proton, 2 neutrons (radioactive).

Q33. A student writes electronic configuration of oxygen as 2,8. Is it correct? Explain.
Ans. No. Oxygen has atomic number 8 → 2,6 is correct.

Q34. A neutral atom has 11 protons. Find its atomic number and electronic configuration.
Ans. Atomic number = 11 → Sodium. Electronic configuration = 2,8,1.

Q35. Why is helium chemically inert?
Ans. Because it has 2 electrons in its only shell (duplet rule satisfied).

Q36. Why is the nucleus positively charged?
Ans. Because it contains protons which are positively charged.

Q37. Why can’t neutron be discovered before 1932?
Ans. Because neutron is neutral, so it leaves no track in electric/magnetic fields.

Q38. Why are protons and neutrons called nucleons?
Ans. Because they are present in the nucleus of an atom.

Q39. Case Study – Carbon Isotopes
Carbon exists as ¹²C, ¹³C, ¹⁴C.
(i) Which one is radioactive?
(ii) Which isotope is used in carbon dating?
Ans.
(i) ¹⁴C.
(ii) ¹⁴C is used in carbon dating.

Q40. What are isoelectronic species? Give examples.
Ans. Species with same number of electrons. Example: Na⁺ (10e⁻), Ne (10e⁻), F⁻ (10e⁻).

🔹 Extra Important (41–50)

Q41. Write the composition of nucleus of lithium atom (Z=3, A=7).
Ans. Protons = 3, Neutrons = 4.

Q42. State the maximum number of electrons in: (i) K shell (ii) L shell.
Ans. (i) 2 (ii) 8.

Q43. What is the valency of chlorine (Z=17)?
Ans. Electronic configuration = 2,8,7 → Valency = 1.

Q44. Why is K shell filled first?
Ans. Because it is nearest to the nucleus and has least energy.

Q45. Write electronic configuration of magnesium (Z=12).
Ans. 2,8,2.

Q46. Define octet rule.
Ans. Atoms tend to attain 8 electrons in their outermost shell to achieve stability.

Q47. Give one application of isotopes in medical field.
Ans. Iodine-131 is used to treat thyroid diseases.

Q48. Give one difference between atomic number and mass number.
Ans.

• Atomic number → Number of protons.
• Mass number → Number of protons + neutrons.

Q49. What is the valency of aluminium (Z=13)?
Ans. 2,8,3 → Valency = 3.

Q50. Why is Na⁺ ion more stable than Na atom?
Ans. Because Na⁺ has 8 electrons in its outermost shell (stable octet), whereas Na atom has only 1.