Structure of Atom Class 9 Question Answer Advanced Science Chapter 6

Students can use NCERT Class 9 Advanced Science Solutions Chapter 6 Structure of Atom Question Answer to understand complex concepts with ease.

Structure of Atom Class 9 Questions and Answers

Structure of Atom Question Answer Class 9

Quick Check

Question 1.
Why do cathode rays bend towards the positive plate?
Answer:
Cathode rays are made of negatively charged particles (electrons). Opposite charges attract each other, so they bend towards the positive plate.

Question 2.
What conclusion did Thomson draw from using different gases in discharge tubes?
Answer:
J. J. Thomson concluded that electrons are present in all atoms, as the cathode rays behaved the same regardless of the gas used.

Question 3.
Why are canal rays different from cathode rays in nature?
Answer:
Canal rays: Positively charged, heavier particles (depend on gas)
Cathode rays: Negatively charged electrons, same for all gases.

Question 4.
Why was the discovery of the neutron necessary?
Answer:
The presence of neutrons explains the mass of atoms. For example, helium contains two protons and two neutrons, which accounts for its mass being approximately four times that of hydrogen.

Question 5.
In a cathode ray experiment, it was observed that the rays bend towards a positively charged plate. What can we conclude about the nature of these rays?
Answer:
It shows that cathode rays are negatively charged particles.

Question 6.
In a discharge tube experiment, the gas is changed from hydrogen to neon, but the behaviour of cathode rays remains unchanged. What does this observation tell us about electrons?
Answer:
By using different gases like neon, it was found that the properties of cathode rays remained unchanged when a gas other than hydrogen was used. This showed that electrons are present in all atoms.

Question 7.
If cathode rays were neutral instead of being negatively charged, how would their behaviour differ in an electric field?
Answer:
They would not be deflected, because only charged particles are affected by electric fields.

Question 8.
In an experiment with canal rays, different gases are used, and different masses of particles are observed. What conclusion can be drawn about the nature of canal rays?
Answer:
It shows that canal rays are made of positively charged particles whose mass depends on the gas, meaning they are not identical for all elements.

Question 9.
Why did scientists feel the need to propose the existence of neutral particles even after discovering electrons and protons? Explain using the example of Helium.
Answer:
Scientists found that the mass of atoms could not be explained by protons and electrons.

Example: Helium has 2 protons and 2 electrons alone. If only these were present, its mass should be very small (~ 2 units from protons, electrons negligible). But when the atomic mass of helium is measured, it is found to be 4 units, which is unexplained. Hence, the scientists proposed the existence of neutral particles (neutrons) in the nucleus that carry no charge but add mass.

Question 10.
In Chadwick’s experiment, the emitted particles were not deflected by electric or magnetic fields. What does this observation indicate about the nature of these particles?
Answer:
In James Chadwick’s experiment, the particles were not deflected by electric or magnetic fields, which indicates that the subatomic particles have no electric charge and are neutral in nature. Hence, these particles were identified as neutrons.

Question 11.
The hydrogen spectrum consists of only a few sharp spectral lines instead of a continuous spectrum. What information does it provide about the energy of electrons in an atom?
Answer:
The hydrogen spectrum has discrete sharp lines, which shows that electrons in an atom have fixed (quantised) energy levels.

Question 12.
Explain why Rutherford’s model would predict a continuous spectrum rather than a line spectrum.
Answer:
In Ernest Rutherford’s model, electrons revolve around the nucleus like planets. According to classical physics, moving electrons should continuously lose energy and emit radiation.

Question 13.
A discharge tube filled with an unknown gas produces a line spectrum identical to hydrogen. What can you conclude about the gas? Give reason.
Answer:
The unknown gas is hydrogen or contains a significant amount of hydrogen gas or hydrogen like species with one electron e.g., He⁺ or Li²⁺.
This is because every element gives a unique line spectrum.

Question 14.
If electrons in an atom were allowed to have a continuous set of energy values, what kind of spectrum would you expect? Why is this not observed?
Answer:
If electrons in an atom were allowed to have a continuous set of energy values, then a continuous spectrum would be observed. But it is not possible because electrons actually have fixed energy levels, not continuous ones.

Question 15.
‘Bohr’s model solved all problems of atomic structure.’
Answer:
Though Bohr’s model was able to explain the stability of an atom and the line spectrum of hydrogen, it failed for multi-electron atoms. In addition, it could not explain fine spectral details.

Question 16.
How does the concept of fixed energy levels explain the stability of atoms?
Answer:
The electrons revolve only in those orbits that have a fixed value of energy. As long as the electron remains in a particular orbit, it neither loses nor gains energy.

Question 17.
Why do different elements produce different line spectra? Give a conceptual explanation.
Answer:
Different elements have different electronic configurations and hence different energy level arrangements. When electrons transition between these energy levels, they emit light of specific wavelengths, producing a unique line spectrum (fingerprint) for each element.

Question 18.
Explain why Bohr’s model works well for hydrogen but not for multi-electron atoms.
Answer:
Niels Bohr’s model works for hydrogen because it has only one electron, so calculations are simple. In multi-electron atoms, electron-electron interactions and repulsions are present, which Bohr’s model cannot explain.

Question 19.
State two limitations of Rutherford’s model.
Answer:
The two limitations of Rutherford’s model are as follows:

1. It could not explain the stability of the atom (electrons should fall into the nucleus).
2. It could not explain line spectra of atoms.

Question 20.
Rutherford’s model explained the structure of the atom but failed to explain atomic stability and spectra. Discuss.
Answer:
Ernest Rutherford’s model of the atom successfully explained that an atom consists of a small, dense and positively charged nucleus at the centre, with electrons revolving around it, and that most of the atom is empty space. This was a major advancement in understanding atomic structure.

However, the model had important limitations.
A moving electron should continuously lose energy in the form of radiation and gradually spiral into the nucleus. This means the atom should collapse, which contradicts the observed stability of atoms.

Additionally, the model could not explain the line spectra of elements. If electrons were losing energy continuously, they would emit a continuous spectrum, but experiments show that atoms produce discrete line spectra.

Thus, while Rutherford’s model explained the basic structure of the atom, it failed to account for atomic stability and the discrete nature of atomic spectra, leading to the development of improved models like Bohr’s model.

Question 21.
What was the main drawback of Rutherford’s model regarding electron motion? What assumption was made by Bohr to overcome this problem?
Answer:
Drawback of Rutherford’s model:
Electrons moving in circular orbits should lose energy continuously and collapse into the nucleus.

Assumption made by Bohr to overcome this problem: Electrons move in fixed energy levels (orbits) where they do not radiate energy because they neither gain nor lose energy.

Question 22.
How does Bohr’s model explain the line spectrum of hydrogen?
Answer:
Whenever electrons move from a higher to a lower level, they emit energy as light of a specific wavelength, producing a line spectrum.

Question 23.
Outline the limitations of Bohr’s model.
Answer:
Limitations of Bohr’s model:

1. Finer details (that is, closely-spaced lines) of the hydrogen atom spectrum observed by sophisticated spectroscopic techniques could not be explained.
2. The spectrum of atoms other than hydrogen could not be explained.
3. The splitting of spectral lines in the presence of a magnetic field (Zeeman effect) or an electric field (Stark effect) could not be explained.

Question 24.
Define line spectrum and continuous spectrum with one example each.
Answer:
Continuous spectrum: A continuous spectrum is one in which all wavelengths of radiation are so intermixed that there is no line of separation between two colours.
For example, sunlight, rainbow, etc.

Line spectrum: A line spectrum is a spectrum that consists of distinct, separate lines of specific wavelengths (or colours), with dark spaces in between, instead of a continuous range of colours.
For example, a sodium vapour lamp and a mercury vapour lamp.

Question 25.
Write two main postulates of Bohr’s model.
Answer:
The two main postulates of Bohr’s model are as follows:

1. Electrons revolve in fixed orbits with definite energy.
2. Energy is emitted or absorbed only when electrons jump between orbits.

Question 26.
What is meant by fine structure in the hydrogen spectrum?
Answer:
Fine structure refers to the splitting of spectral lines into very closely spaced lines, due to subtle effects like electron spin and relativistic corrections.

Question 27.
What is the significance of the Rydberg equation?
Answer:
The Rydberg equation is used to calculate the wavelength of spectral lines in the hydrogen spectrum.

Structure of Atom Class 9 Extra Questions and Answers

Short Answer Type Questions

Question 1.
Give any two failures of Bohr’s atomic model.
Answer:

1. It could not explain the spectrum of multielectron atoms.
2. It could not explain the splitting of spectral lines in the presence of a magnetic field (Zeeman effect) and an electric field (Stark effect).

Question 2.
Which series of lines of the hydrogen spectrum lies in the visible region?
Answer:
The Balmer series of the hydrogen spectrum lies in the visible region of the electromagnetic spectrum.
In this series, electrons fall from higher energy levels (n = 3, 4, 5…) to the second energy level (n = 2), producing visible spectral lines.

Question 3.
Why are spectral lines obtained in hydrogen spectrum?
Answer:
Spectral lines are obtained because electrons move between fixed energy levels and emit specific amounts of energy.

Question 4.
Give the essential postulates of Bohr’s model of an atom. How did it explain:
(i) The stability of the atom
(ii) Origin of the spectral lines in H-atom?
Answer:
Postulates of Bohr’s model: The electron in the atom can move around the nucleus in a circular path of fixed radius and energy. These paths are called orbits, stationary states, or allowed energy states. These orbits are arranged concentrically around the nucleus. Radiation can occur only when the electron jumps from one orbit to another.

(i) The atom will be stable because electrons are restricted to these stationary orbits and cannot loose energy continuously, they do not spiral into the nucleus.

(ii) According to Bohr’s model, radiation (energy) is absorbed if the electron moves from the orbit of a smaller principal quantum number to the orbit of a higher principal quantum number, whereas the radiation (energy) is emitted if the electron moves from a higher” orbit to a lower orbit. It explains the origin of spectral lines in the H-atom.

Long Answer Type Questions

Question 1.
Explain Bohr’s atomic model and how it explains the stability of atoms.
Answer:
Niels Bohr improved Rutherford’s model by introducing the concept of fixed energy levels.
Postulates of Bohr’s Model:

• Electrons revolve around the nucleus only in certain fixed circular paths called orbits or energy levels.
• Electrons moving in these fixed orbits do not lose energy.
• Energy is emitted or absorbed only when electrons jump from one energy level to another.

Explanation of Atomic Stability:
According to Bohr, electrons in fixed energy levels do not radiate energy continuously. Therefore, they do not spiral into the nucleus, making the atom stable.

Case-Based MCQs

I. A scientist performed experiments using a discharge tube containing gases at very low pressure. When high voltage was applied, rays were produced from the cathode and m’oved towards the anode. These rays were deflected towards the positive plate in an electric field.

Question 1.
The rays produced in the experiment are called:
(A) Canal rays
(B) Alpha rays
(C) Cathode rays
(D) X-rays
Answer:
Option (C) is correct.
Explanation: The rays originating from the cathode in a discharge tube are called cathode rays.

Question 2.
Deflection towards the positive plate shows that cathode rays are:
(A) Neutral
(B) Positively charged
(C) Negatively charged
(D) Radioactive 0
Answer:
Option (C) is correct.
Explanation: Opposite charges attract each other. Since cathode rays bend towards the positive plate, they must be negatively charged.

Question 3.
What conclusion did scientists draw when different gases showed the same behaviour of cathode rays?
(A) Electrons are present only in hydrogen
(B) Electrons are present in all atoms
(C) Cathode rays are neutral
(D) Protons are absent in atoms
Answer:
Option (B) is correct.
Explanation: The identical behaviour of cathode rays with different gases proved that electrons are common to all atoms.

Question 4.
The nucleus of helium contains:
(A) Four protons
(B) Four neutrons
(C) Two neutrons and two protons
(D) Four protons and two electrons
Answer:
Option (C) is correct.
Explanation: A helium atom has an atomic number 2, which means it contains 2 protons. Its most common isotope has a mass number of 4, so it also contains 2 neutrons in the nucleus. Therefore, the nucleus of helium consists of 2 protons and 2 neutrons.

Case-Based Subjective Questions

I. A science teacher showed students a glowing neon sign and asked why different gases emit different colours of light. The teacher explained that according to Bohr’s Model of the Atom, electrons revolve around the nucleus in fixed energy levels or shells. When electrons gain energy, they jump to a higher shell, and when they return to a lower shell, they release energy in the form of light. Riya observed that hydrogen gas emits specific coloured lines when viewed through a spectroscope. She learned that this happens because electrons move between fixed energy levels.

Question 1.
According to Bohr’s model, where are electrons present in an atom?
Answer:
According to Bohr’s model, electrons revolve around the nucleus in fixed circular paths called shells or energy levels.

Question 2.
What happens when an electron absorbs energy?
Answer:
When an electron absorbs energy, it jumps from a lower energy level to a higher energy level, called an” excited state.

Question 3.
Why does an atom emit light when electrons return to lower energy levels?
Answer:
When electrons return from a higher energy level to a lower energy level, they release excess energy in the form of light.

Question 4.
Explain why hydrogen shows only specific spectral lines and not a continuous spectrum.
Answer:
Hydrogen shows only specific spectral lines because electrons can exist only in fixed energy levels. They emit only certain amounts of energy while moving between these levels, producing specific lines instead of a continuous spectrum.

Structure of Atom Class 9 MCQ

Question 1.
Which type of spectrum contains all colours without any gaps?
(A) Line spectrum
(B) Emission spectrum
(C) Continuous spectrum
(D) Absorption spectrum
Answer:
Option (C) is correct.
Explanation: A continuous spectrum contains all wavelengths or colours without breaks, like the spectrum formed by sunlight.

Question 2.
Which scientist explained the hydrogen line spectrum using fixed orbits?
(A) Ernest Rutherford
(B) J. J. Thomson
(C) Niels Bohr
(D) James Chadwick
Answer:
Option (C) is correct.
Explanation: Bohr proposed that electrons revolve in fixed energy levels without losing energy, explaining the hydrogen line spectrum.

Question 3.
Rutherford’s model could not explain the following:
(A) Presence of nucleus
(B) Empty space in atom
(C) Stability of atom
(D) Positive charge of nucleus
Answer:
Option (C) is correct.
Explanation: According to Rutherford’s model, moving electrons should lose energy and fall into the nucleus, making atoms unstable.

Question 4.
Why did Rutherford’s model predict a continuous spectrum?
(A) Electrons had fixed energies
(B) Electrons continuously lost energy
(C) Nucleus emitted light
(D) Protons absorbed energy
Answer:
Option (B) is correct.
Explanation: According to classical theory, revolving electrons should continuously emit energy, producing a continuous spectrum.

Assertion-Reason Questions

Directions: In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Mark the correct choice as:
(A) Both Assertion (A) and Reason (R) are true, and Reason (R) is correct ex¬planation of Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true.

Question 1.
Assertion (A): Zeeman effect is the splitting of a spectral line in the presence of a magnetic field.
Reason (R): Stark effect is the splitting of the spectral line in the presence of an electric field.
Answer:
Option (B) is correct.
Explanation: The assertion is correct because the Zeeman effect refers to the splitting of spectral lines when an atom is placed in the presence of a magnetic field. The reason is also correct because the Stark effect refers to the splitting of spectral lines in the presence of an electric field. However, the reason does not explain the assertion, since the Zeeman effect and Stark effect are two separate phenomena caused by different external fields.

Question 2.
Assertion (A): Different elements produce different spectra.
Reason (R): Different elements have different electronic arrangements.
Answer:
Option (A) is correct.
Explanation: Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion. Different elements produce different spectra due to different electronic configuration.