Let’s Revise the Topics Covered under this Chapter 4 Structure of Atom: [For detailed Understanding, visit our NOTES by CLICK HERE]
1. CHARGED PARTICLES IN MATTER
4. Atomic Number and Mass Number
5. Electron Distribution in Orbits
Test your understanding with these important questions designed for Class 9 Science Chapter 4 – Structure of Atom. Practice them carefully to strengthen your concepts and secure full marks in your Exam.
Short Answer Type Questions:
Q1. Why is the atomic number more important than the mass number in identifying an element?
Ans: The atomic number represents the number of protons in an atom, and it is constant for each element. Because chemical properties are determined by the amount of electrons, which are equivalent to protons in a neutral atom, the atomic number determines both the element’s identity and behaviour. However, the mass number of isotopes might differ due to differing neutrons.
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Q2. Why are isotopes useful in medicine and industry despite having the same chemical properties?
Ans: Isotopes have equal chemical properties, but their different masses and nuclear stabilities make them useful. For example, radioactive isotopes such as cobalt-60 are used in cancer treatment, whereas iodine-131 is used to detect thyroid diseases. Their physical distinctions, particularly radioactivity, make them useful in medicine and industry.
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Q3. How does Bohr’s model explain the stability of atoms which Rutherford’s model could not?
Ans: Rutherford’s model could not explain why electrons that orbit the nucleus do not lose energy and fall into it. Bohr answered this problem by arguing that electrons only move in fixed orbits or energy levels in which they do not emit energy. This quantised shell notion successfully explained atomic stability.
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Q4. Why are isotopes of hydrogen (protium, deuterium, tritium) considered special?
Ans: Hydrogen is unique its isotopes differ not just in neutron count, but also in mass and nuclear stability. Protium has no neutrons, deuterium has one, and tritium, which has two neutrons, is radioactive. Hydrogen isotopes are extremely important in nuclear science and research due to their huge differences within a single element.
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Q5. Why is the nucleus described as both very small and very dense?
Ans: The nucleus is just approximately 10⁻¹⁵ meters in size, yet it contains nearly the whole mass of the atom in the form of protons and neutrons. The nucleus is known as “very dense” because it contains so much mass in such a small volume. This also explains why nuclear particles have such incredible power.
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Q6. Why is the discovery of the neutron considered the last step in completing the basic atomic structure?
Ans: Prior to the discovery of the neutron, scientists knew about protons and electrons, but atomic masses could not be adequately described. Chadwick discovered the neutron, which explained the nucleus’s lacking mass and stability. This completed the basic structure of the atom, which consisted of protons, neutrons, and electrons.
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Q7. Why can isotopes be separated by physical methods but not by chemical methods?
Ans: Isotopes have similar electron configurations, therefore they exhibit the same chemical properties and cannot be separated by chemical reactions. However, their mass differences allow for separation using physical means such as diffusion or mass spectrometry. Thus, chemical procedures fail while physical strategies develop.
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Q8. Why is the electron considered the most significant particle in chemical bonding?
Ans: Chemical reactions are caused by electrons, which are found in the outer shells of atoms despite being extremely light. For chemical bonds to be stable, these electrons must be shared or transferred. Thus, the most important component in bonding is electrons, particularly valence electrons.
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Q9. How did the discovery of isotopes challenge Dalton’s Atomic Theory?
Ans: According to Dalton’s hypothesis, atoms of the same element share the same mass and characteristics. However, the discovery of isotopes demonstrated that atoms of the same element might have identical chemical properties but differ in mass. This amended Dalton’s theory and led to our current understanding of atomic structure.
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Q10. Why are protons and neutrons together called nucleons?
Ans: Protons and neutrons are referred to as nucleons because they are present within the nucleus of an atom. Both contribute nearly the entire atomic mass and are bound together by powerful nuclear forces. Nucleons are collectively referred to as such since they form the atom’s core.
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Q11. Why does the atomic radius largely depend on the arrangement of electrons rather than the nucleus?
The nucleus is quite small and remains constant in size for each element, but the distribution of electrons in shells dictates how much space an atom occupies. The size of new shells grows when electrons are supplied, and reduces when more nuclear charge pulls them closer together. As a result, the electron configuration has the greatest impact on atomic radius.
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Q12. Why do isotopes of the same element show identical chemical properties?
Ans: Chemical properties are determined by the number of electrons in an element’s outermost shell, which is constant across all isotopes. For example, carbon-12 and carbon-14 both have six protons and six electrons, resulting in the identical chemical behaviour. The difference between neutrons solely impacts their physical and nuclear properties, not their chemical ones.
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Q13. How did Thomson’s model of the atom differ from Rutherford’s model?
Ans: Thomson’s concept proposed that positive charge was distributed uniformly, with electrons embedded in it, like ‘plum pudding’. Rutherford’s work contradicted this by demonstrating that the positive charge is concentrated in a very small nucleus at the centre. Thus, Rutherford’s nuclear model supplanted Thomson’s model and established the foundation of current atomic structure.
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Q14. Why is the study of atomic models important in understanding modern chemistry?
Ans: Each atomic model, from Dalton to Bohr, contributed incrementally to our current understanding of the atom. These models explored ideas such as indivisibility, the existence of subatomic particles, nuclear structure, and quantised energy levels. Modern chemistry and understanding of bonding, reactions, and atomic behaviour would not have been feasible if atomic models had not gradually evolved over time.
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Q15. Why are neutrons sometimes called the “nuclear cement”?
Ans: Neutrons serve to bind protons together within the nucleus. Protons oppose each other due to their positive charge, but the strong nuclear force generated by neutrons balances this repulsion and binds the nucleus together. The nucleus would be unstable without neutrons, therefore they act as cement in the atomic structure.
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Q16. How does the discovery of cathode rays prove the existence of electrons?
Ans: In the cathode ray experiment, a stream of particles was observed moving from the negative electrode to the positive electrode in a vacuum tube. These rays were deflected by electric and magnetic forces, indicating a negative charge. Later, these particles were identified as electrons, indicating the presence of subatomic particles within atoms.
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Q17. Why is Bohr’s model still used in basic chemistry although it has limitations?
Ans: Bohr’s model, while not entirely right for complicated atoms, is extremely useful in explaining the structure of basic atoms such as hydrogen. It introduces the concepts of shells, energy levels, and electron transitions, all of which are relevant to understanding atomic spectra and bonds. As a result, it is still taught as a foundation, even though current quantum theory has taken its place.
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Q18. Why does the atomic mass of an element often appear as a fractional number?
Ans: An element’s atomic mass is calculated as the weighted average of its naturally occurring isotope masses. For example, chlorine has two isotopes, Cl-35 and Cl-37, and its average atomic mass is approximately 35.5. Because it is an average value, it frequently appears as a fractional number.
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Q19. How does the presence of neutrons affect the stability of nuclei?
Ans: Neutrons reduce the strong repulsive attraction between protons by supplying more nuclear force without introducing charge. This helps to stabilise the nucleus. If there are too few or too many neutrons, the nucleus becomes unstable and radioactive, demonstrating the importance of neutrons in nuclear stability.
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Q20. Why are electrons arranged in shells around the nucleus instead of staying close to it?
Ans: Electrons are negatively charged and drawn to the positively charged nucleus, but they also carry energy that propels them forward. To maintain stability, electrons inhabit specified energy levels or shells at various distances from the nucleus. These shells prevent electrons from falling into the nucleus and help to explain atomic structure.
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Class 9 Important Question Answers
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HIGH LEVEL THINKING QUESTIONS
Q1. If atoms are mostly empty space, why does matter appear solid to us?
Ans: Although an atom is mostly empty space, the outer region is occupied by electrons which form an electron cloud. When two objects are brought close, their electron clouds strongly repel each other due to electrostatic forces. This repulsion prevents atoms from passing through one another, making matter appear solid.
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Q2. Why was Bohr’s model considered an improvement over Rutherford’s model?
Ans: Rutherford’s model explained the nucleus but could not explain why electrons do not collapse into it. Bohr improved this by giving the concept of quantized energy levels.
- Electrons revolve in fixed energy shells without radiating energy.
- Energy is absorbed or emitted only when electrons jump between shells.
- This explained the stability of atoms and the atomic spectra.
Thus, Bohr’s model was a significant advancement over Rutherford’s.
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Q3. Why is the neutron called the “nuclear glue”?
Ans: Inside the nucleus, positively charged protons repel each other strongly. Neutrons, though electrically neutral, provide strong nuclear forces that bind protons and neutrons together.
- Reduce repulsion between protons.
- Provide stability to the nucleus.
- Hold nucleons tightly together.
Hence, neutrons are called the “nuclear glue.”
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Q4. How did the discovery of isotopes modify Dalton’s Atomic Theory?
Ans: Dalton stated that atoms of the same element are identical in all respects. However, the discovery of isotopes proved this wrong because:
- Isotopes of an element have the same atomic number but different mass numbers.
- They show identical chemical properties but differ in physical properties.
Thus, isotopes showed that Dalton’s theory was incomplete and needed modification.
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Q5. Why is atomic number considered more fundamental than atomic mass in identifying an element?
Ans: Atomic number represents the number of protons in an atom. Since each element has a unique number of protons, atomic number is its true identity.
- Atomic number determines electron arrangement.
- Electron arrangement decides chemical properties.
- Atomic mass may vary due to isotopes, but atomic number remains constant.
Therefore, atomic number is more fundamental than atomic mass.
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Q6. Why is the atomic mass of chlorine not a whole number but 35.5?
Ans: Chlorine occurs in nature mainly as two isotopes – Cl-35 and Cl-37.
- Cl-35 has 75% abundance.
- Cl-37 has 25% abundance.
The average atomic mass = (35 × 75/100) + (37 × 25/100) = 35.5
Thus, chlorine’s atomic mass is a fractional value, not a whole number.
Q7. Why are electrons considered most important for chemical bonding?
Ans: The chemical properties of an element depend on its valence electrons.
- Atoms combine by losing, gaining, or sharing valence electrons.
- This electron transfer or sharing forms chemical bonds.
- Stable configurations (like noble gases) are achieved through valence electrons.
Hence, electrons are the most important particles in chemical bonding.
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Q8. How does the discovery of cathode rays prove atoms are divisible?
Ans: Dalton considered atoms to be indivisible. However, cathode ray experiments showed the existence of negatively charged particles called electrons.
- Cathode rays were deflected by electric and magnetic fields.
- They carried negative charge and very small mass.
- This proved atoms consist of smaller sub-atomic particles.
Thus, atoms are divisible and not the ultimate indivisible units.
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Q9. Why does the nucleus of an atom resemble the “sun” and electrons resemble “planets”?
Ans: The nucleus is small, dense, and positively charged, like the sun at the center of the solar system. Electrons revolve around the nucleus just as planets revolve around the sun.
- Nucleus: massive and central.
- Electrons: move in definite paths around it.
- Attractive forces keep electrons bound like gravity binds planets.
This analogy explains the atomic structure.
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Q10. What would happen if electrons were heavier than protons?
Ans: The balance of atomic structure would be completely disturbed if electrons were heavier than protons.
- Electrons would dominate the atom’s mass.
- Orbiting of electrons would not be possible due to excess mass.
- Atoms would become unstable and lose their present structure.
- Chemical reactions and bonding as we know them would not exist.
Thus, matter and life would be impossible in such a case.
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