Chapter 4: Structure of the Atom Long Questions | science Class 9 CBSE

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Chapter 4: Structure of the Atom Long Questions | science Class 9 CBSE | ByteNotes.in

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Question 1

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Q1. Describe the discovery of electrons and protons in atoms.

• J.J. Thomson discovered the electron in 1897 through cathode ray experiments.
• Electrons are negatively charged sub-atomic particles, represented as 'e⁻'.
• E. Goldstein discovered canal rays in 1886, which led to the discovery of protons.
• Protons are positively charged particles, represented as 'p⁺'.
• Mass of a proton is approximately 2000 times that of an electron.
• Electrons can be removed from atoms easily, but protons are located in the atom’s interior.
• These discoveries proved that atoms are divisible and consist of charged sub-atomic particles.

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Question 2

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Q2. Explain Thomson’s model of the atom.

• Thomson proposed the 'plum pudding model' or 'watermelon model'.
• Atom consists of a positively charged sphere with electrons embedded like seeds in a watermelon.
• Positive and negative charges are equal in magnitude, making the atom electrically neutral.
• This model could explain electrical neutrality of atoms.
• However, it failed to explain results of later experiments such as Rutherford’s scattering experiment.
• No explanation of atomic stability or structure of nucleus was provided.

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Question 3

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Q3. Describe Rutherford’s gold foil experiment and the conclusions drawn from it.

• Rutherford bombarded fast-moving alpha (α) particles on a thin gold foil (about 1000 atoms thick).
• Observations: Most α-particles passed straight through; some were deflected; very few rebounded.
• Conclusions:
– Most of the atom is empty space.
– A small positively charged region called nucleus is at the center.
– Nucleus contains nearly all the atom’s mass.
– Electrons revolve around the nucleus.
• Nucleus radius is about 1/100,000 of the atom’s radius.

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Question 4

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Q4. What were the limitations of Rutherford’s model of the atom?

• Rutherford proposed that electrons revolve around the nucleus in circular orbits.
• According to electromagnetic theory, a charged particle undergoing acceleration emits energy.
• Revolving electrons would emit energy and spiral into the nucleus.
• This would make the atom unstable, contradicting the observed stability of matter.
• The model failed to explain atomic spectra and electron arrangement.
• It could not describe behavior of electrons in atoms beyond hydrogen.

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Question 5

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Q5. State the postulates of Bohr’s model of the atom.

• Electrons revolve around the nucleus in certain fixed circular orbits (energy levels).
• These orbits are also called shells or energy levels and are represented by K, L, M, N or n = 1, 2, 3, 4.
• While revolving in a particular orbit, electrons do not radiate energy.
• Energy is emitted or absorbed only when electrons jump between energy levels.
• This model explained stability of atom and atomic spectra.
• Bohr’s model is applicable mainly to hydrogen-like atoms.

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Question 6

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Q6. What are neutrons? Who discovered them and what are their properties?

• Neutrons are neutral sub-atomic particles present in the nucleus.
• They were discovered by James Chadwick in 1932.
• Neutrons have no charge and mass nearly equal to that of protons.
• Represented as 'n'.
• Neutrons are present in all atoms except ordinary hydrogen.
• They contribute to the mass of an atom.
• Neutrons, along with protons, are called nucleons.

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Question 7

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Q7. How are electrons distributed in different shells around the nucleus?

• Distribution is based on Bohr-Bury rules:
(i) Maximum electrons in a shell = 2n² (n = shell number).
– K-shell (n=1): 2 electrons
– L-shell (n=2): 8 electrons
– M-shell (n=3): 18 electrons
– N-shell (n=4): 32 electrons
(ii) Outermost shell can have a maximum of 8 electrons.
(iii) Electrons fill inner shells first, step-by-step.
• For example: Oxygen (8 electrons) → 2 in K, 6 in L.

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Question 8

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Q8. What is valency? How is it calculated using electronic configuration?

• Valency is the combining capacity of an atom.
• It depends on the number of valence electrons (electrons in the outermost shell).
• If the outer shell has 1–4 electrons, valency = number of valence electrons.
• If the outer shell has 5–7 electrons, valency = 8 – number of valence electrons.
• If outer shell has 8 electrons, valency = 0 (inert elements).
• Examples:
– Sodium (2,8,1): valency = 1
– Oxygen (2,6): valency = 2
– Chlorine (2,8,7): valency = 1

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Question 9

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Q9. Define atomic number and mass number. Explain with examples.

• Atomic number (Z): Number of protons in the nucleus of an atom.
• It is also equal to the number of electrons in a neutral atom.
• Example: Carbon (Z = 6) has 6 protons and 6 electrons.
• Mass number (A): Total number of protons and neutrons in the nucleus.
• Example: Oxygen (8 protons + 8 neutrons) → A = 16
• Denotation: A/Z Element Symbol (e.g., 16/8 O).
• Mass number = protons + neutrons

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Question 10

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Q10. What are isotopes? Explain with examples and mention any two applications.

• Isotopes are atoms of the same element having the same atomic number but different mass numbers.
• Example: Hydrogen has 3 isotopes – Protium (¹H), Deuterium (²H), Tritium (³H).
• Chlorine: ³⁵Cl and ³⁷Cl with atomic number 17, but mass numbers 35 and 37.
• Isotopes have same chemical properties but different physical properties.
• Applications:
– Isotope of cobalt used in cancer treatment.
– Isotope of iodine used in goitre treatment.
– Isotope of uranium used in nuclear reactors.

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Chapter 4: Structure of the Atom | Long Questions

Q1. Describe the discovery of electrons and protons in atoms.

Q2. Explain Thomson’s model of the atom.

Q3. Describe Rutherford’s gold foil experiment and the conclusions drawn from it.

Q4. What were the limitations of Rutherford’s model of the atom?

Q5. State the postulates of Bohr’s model of the atom.

Q6. What are neutrons? Who discovered them and what are their properties?

Q7. How are electrons distributed in different shells around the nucleus?

Q8. What is valency? How is it calculated using electronic configuration?

Q9. Define atomic number and mass number. Explain with examples.

Q10. What are isotopes? Explain with examples and mention any two applications.

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