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Structure of the Atom – CBSE NCERT Study Resources

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Structure of the Atom

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9th

9th - Science

Structure of the Atom

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Structure of the Atom

Chapter Overview

This chapter explores the fundamental concepts related to the structure of an atom, including subatomic particles, atomic models, and the arrangement of electrons in atoms. It builds upon the basic understanding of matter and its composition, leading to a deeper insight into chemical behavior.

An atom is the smallest unit of matter that retains the properties of an element. It consists of a nucleus (protons and neutrons) surrounded by electrons.

Subatomic Particles

Atoms are composed of three primary subatomic particles:

  • Protons - Positively charged particles found in the nucleus.
  • Neutrons - Neutral particles found in the nucleus.
  • Electrons - Negatively charged particles orbiting the nucleus.

The atomic number (Z) is the number of protons in an atom, which determines the element's identity.

Atomic Models

Several models have been proposed to explain the structure of the atom:

  • Thomson's Model (Plum Pudding Model) - Suggested that atoms are spheres of positive charge with electrons embedded in them.
  • Rutherford's Model - Proposed a dense nucleus with electrons orbiting around it, based on the gold foil experiment.
  • Bohr's Model - Introduced the concept of fixed electron orbits (energy levels) around the nucleus.

Electron Distribution

Electrons are arranged in shells or energy levels around the nucleus. The distribution follows the formula 2n², where n is the shell number.

The valency of an element is determined by the number of electrons it can gain, lose, or share to achieve a stable electron configuration.

Isotopes and Isobars

Atoms of the same element with different mass numbers are called isotopes (e.g., Hydrogen-1, Hydrogen-2). Isobars are atoms of different elements with the same mass number (e.g., Calcium-40 and Argon-40).

All Question Types with Solutions – CBSE Exam Pattern

Explore a complete set of CBSE-style questions with detailed solutions, categorized by marks and question types. Ideal for exam preparation, revision and practice.

Very Short Answer (1 Mark) – with Solutions (CBSE Pattern)

These are 1-mark questions requiring direct, concise answers. Ideal for quick recall and concept clarity.

Question 1:
What is the charge of an electron?
Answer:

The electron has a negative charge.

Question 2:
Name the subatomic particle with no charge.
Answer:

The neutron has no charge.

Question 3:
Who discovered the nucleus of the atom?
Answer:

Ernest Rutherford discovered the nucleus.

Question 4:
What is the atomic number of Hydrogen?
Answer:

The atomic number of Hydrogen is 1.

Question 5:
Which model proposed that electrons move in fixed orbits?
Answer:

Bohr's model proposed fixed orbits.

Question 6:
What is the mass number of an atom with 6 protons and 6 neutrons?
Answer:

The mass number is 12.

Question 7:
Name the isotope of Hydrogen with no neutrons.
Answer:

Protium has no neutrons.

Question 8:
What is the valence shell?
Answer:

The outermost shell of an atom.

Question 9:
Who proposed the plum pudding model?
Answer:

J.J. Thomson proposed it.

Question 10:
What is the charge of a proton?
Answer:

The proton has a positive charge.

Question 11:
How many electrons can the first shell hold?
Answer:

The first shell holds 2 electrons.

Question 12:
What is an ion?
Answer:

An atom with a net charge.

Question 13:
Name the element with atomic number 2.
Answer:

Helium has atomic number 2.

Question 14:
What is the symbol for Gold?
Answer:

The symbol for Gold is Au.

Question 15:
What is the atomic number of an element?
Answer:

The atomic number of an element is the number of protons present in the nucleus of its atom. It determines the identity of the element.

Question 16:
Name the subatomic particle with no charge.
Answer:

The neutron is the subatomic particle with no charge (neutral). It is located in the nucleus.

Question 17:
What is an isotope?
Answer:

Isotopes are atoms of the same element with the same atomic number but different mass numbers due to varying numbers of neutrons.

Question 18:
Who discovered the electron?
Answer:

J.J. Thomson discovered the electron in 1897 through his cathode ray experiment.

Question 19:
What is the valency of an atom?
Answer:

Valency is the combining capacity of an atom, determined by the number of electrons it can lose, gain, or share to achieve a stable configuration.

Question 20:
Which subatomic particle determines the mass of an atom?
Answer:

The protons and neutrons in the nucleus determine the mass of an atom, as electrons have negligible mass.

Question 21:
What is the electronic configuration of Oxygen (Atomic Number = 8)?
Answer:

The electronic configuration of Oxygen is 2, 6.
First shell (K): 2 electrons
Second shell (L): 6 electrons

Question 22:
Define nucleons.
Answer:

Nucleons are the particles present in the nucleus of an atom, which include protons and neutrons.

Question 23:
What is the charge on an electron?
Answer:

An electron carries a negative charge of -1.6 × 10-19 Coulombs.

Question 24:
How many electrons can the M shell hold?
Answer:

The M shell (n=3) can hold a maximum of 18 electrons as per the formula 2n2.

Question 25:
What is the mass number of an atom?
Answer:

The mass number is the sum of protons and neutrons in the nucleus of an atom.

Question 26:
Name the scientist who proposed the planetary model of the atom.
Answer:

Ernest Rutherford proposed the planetary model of the atom, where electrons revolve around the nucleus like planets around the sun.

Very Short Answer (2 Marks) – with Solutions (CBSE Pattern)

These 2-mark questions test key concepts in a brief format. Answers are expected to be accurate and slightly descriptive.

Question 1:
Define isotopes with an example.
Answer:

Isotopes are atoms of the same element with the same atomic number but different mass numbers due to varying numbers of neutrons.
Example: Hydrogen has three isotopes - Protium (1H), Deuterium (2H), and Tritium (3H).

Question 2:
What is the charge and mass of an electron?
Answer:

An electron has a charge of -1.6 × 10⁻¹⁹ C (negative) and a mass of 9.1 × 10⁻³¹ kg, which is negligible compared to protons and neutrons.

Question 3:
State the Rutherford's atomic model.
Answer:

Rutherford's model proposed that:
1. An atom has a tiny, dense, positively charged nucleus.
2. Electrons revolve around the nucleus in circular paths.
3. Most of the atom is empty space.

Question 4:
Why are atoms electrically neutral?
Answer:

Atoms are electrically neutral because the number of protons (positive charge) equals the number of electrons (negative charge), canceling out the charges.

Question 5:
What are valence electrons?
Answer:

Valence electrons are the electrons present in the outermost shell of an atom. They determine the chemical properties and reactivity of the element.

Question 6:
Explain the term mass number.
Answer:

The mass number is the sum of protons and neutrons in an atom's nucleus. It is denoted by A.

Question 7:
What is the Bohr's model of an atom?
Answer:

Bohr's model states that:
1. Electrons move in fixed orbits or shells around the nucleus.
2. Each shell has a definite energy level.
3. Electrons can jump between orbits by absorbing or emitting energy.

Question 8:
How many electrons can the K-shell hold?
Answer:

The K-shell (innermost shell) can hold a maximum of 2 electrons.

Question 9:
What is the difference between an atom and an ion?
Answer:

An atom is electrically neutral, while an ion is a charged particle formed by gaining or losing electrons.
Example: Na (atom) vs. Na⁺ (ion).

Question 10:
Name the subatomic particles present in an atom.
Answer:

The three subatomic particles are:
1. Protons (positively charged)
2. Neutrons (neutral)
3. Electrons (negatively charged).

Short Answer (3 Marks) – with Solutions (CBSE Pattern)

These 3-mark questions require brief explanations and help assess understanding and application of concepts.

Question 1:
Define atomic number and mass number. How are they represented for an element?
Answer:

The atomic number (Z) is the number of protons in the nucleus of an atom. It determines the identity of the element.
The mass number (A) is the sum of protons and neutrons in the nucleus.
They are represented as: ElementAZ. For example, Carbon-12 is written as C126.

Question 2:
Explain why an atom is electrically neutral despite having charged particles.
Answer:

An atom is electrically neutral because the number of protons (positively charged) equals the number of electrons (negatively charged).
The charges balance each other, resulting in no net charge.
For example, a neutral carbon atom has 6 protons and 6 electrons.

Question 3:
Differentiate between valence electrons and valency with examples.
Answer:

Valence electrons are the electrons in the outermost shell of an atom.
Valency is the combining capacity of an atom, determined by the number of electrons it can lose, gain, or share.

  • Example: Oxygen has 6 valence electrons but a valency of 2 (needs 2 more electrons to complete its octet).
  • Sodium has 1 valence electron and a valency of 1 (can lose 1 electron).

Question 4:
Describe the Rutherford's alpha-particle scattering experiment and its conclusions.
Answer:

Rutherford shot alpha particles at a thin gold foil.
Most particles passed through, but some were deflected, and a few bounced back.
Conclusions:

  • Atoms have a small, dense, positively charged nucleus.
  • Most of the atom is empty space.
  • Electrons move around the nucleus.

Question 5:
What are isotopes? Give two examples with their uses.
Answer:

Isotopes are atoms of the same element with the same atomic number but different mass numbers.

  • Carbon-12 and Carbon-14: C-14 is used in carbon dating.
  • Hydrogen-1 and Hydrogen-2 (Deuterium): Deuterium is used in nuclear reactors.

Question 6:
Explain the Bohr's model of the atom and its limitations.
Answer:

Bohr's model states that electrons move in fixed orbits (shells) around the nucleus, each with a definite energy level.
Limitations:

  • It only works for hydrogen-like atoms.
  • Does not explain the fine spectrum of elements.
  • Does not account for electron wave-particle duality.

Question 7:
Explain the Thomson's model of the atom. Why was it later discarded?
Answer:

Thomson's model proposed that an atom is a positively charged sphere with electrons embedded in it, like plums in a pudding.
It was discarded because it couldn't explain:

  • The deflection of alpha particles in Rutherford's experiment.
  • The existence of a dense, positively charged nucleus.

Question 8:
What are isotopes? Give one example with a diagrammatic representation.
Answer:

Isotopes are atoms of the same element with the same atomic number but different mass numbers due to varying neutrons.
Example: Hydrogen has three isotopes:

  • Protium (¹H)
  • Deuterium (²H)
  • Tritium (³H)

[Diagram: Three hydrogen atoms with 0, 1, and 2 neutrons respectively.]

Question 9:
Describe the Rutherford's alpha-particle scattering experiment. What were its key observations?
Answer:

Rutherford fired alpha particles at a thin gold foil. Key observations:

  • Most particles passed straight, indicating empty space.
  • Some deflected slightly, suggesting a positive charge.
  • A few rebounded, proving a dense nucleus.
This led to the nuclear model of the atom.

Question 10:
How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Answer:

The electronic configuration determines:

  • Period number = Number of shells (e.g., Carbon (2,4) is in Period 2).
  • Group number = Valence electrons (e.g., Carbon is in Group 14).
This shows periodic trends like reactivity and atomic size.

Long Answer (5 Marks) – with Solutions (CBSE Pattern)

These 5-mark questions are descriptive and require detailed, structured answers with proper explanation and examples.

Question 1:
Explain the Rutherford's alpha-particle scattering experiment and its conclusions about the structure of the atom.
Answer:
Concept Overview

Rutherford's experiment involved firing alpha particles at a thin gold foil to study atomic structure.

Process Explanation
  • Most particles passed straight, indicating empty space.
  • Some deflected, suggesting a dense, positively charged nucleus.
Real-world Application

This model explains why electrons orbit the nucleus, similar to planets around the sun.

[Diagram: Gold foil experiment setup]
Question 2:
Describe the Bohr's model of the atom and how it improved upon Rutherford's model.
Answer:
Concept Overview

Bohr proposed fixed orbits for electrons to explain atomic stability.

Process Explanation
  • Electrons move in specific energy levels without radiating energy.
  • Explained hydrogen's line spectrum, unlike Rutherford's model.
Real-world Application

Used in understanding atomic spectra, like neon lights.

[Diagram: Bohr's atomic model with orbits]
Question 3:
What are isotopes? Give one NCERT example and a real-world application.
Answer:
Concept Overview

Isotopes are atoms of the same element with different neutron numbers.

Process Explanation
  • Example: Carbon-12 and Carbon-14 (NCERT).
  • Same protons but varying neutrons.
Real-world Application

Carbon-14 is used in radiocarbon dating of fossils.

[Diagram: Isotopes of carbon]
Question 4:
Explain how J.J. Thomson's cathode ray experiment led to the discovery of electrons.
Answer:
Concept Overview

Thomson used cathode rays to identify negatively charged particles.

Process Explanation
  • Rays deflected toward positive plate, proving electrons exist.
  • Led to the 'plum pudding' atomic model.
Real-world Application

This discovery is foundational for electronics like TVs.

[Diagram: Cathode ray tube setup]
Question 5:
Differentiate between valence electrons and kernel electrons with an NCERT example.
Answer:
Concept Overview

Valence electrons are outer-shell electrons involved in bonding, while kernel electrons are inner-shell.

Process Explanation
  • Example: Sodium (NCERT) has 1 valence electron and 10 kernel electrons.
Real-world Application

Valence electrons determine chemical reactivity, like in salt formation.

[Diagram: Sodium atom electron distribution]
Question 6:
Describe the Bohr's model of the atom and how it improves upon Rutherford's model.
Answer:
Concept Overview

Bohr proposed that electrons move in fixed orbits or shells around the nucleus, each with a specific energy level.

Process Explanation
  • Electrons don't radiate energy in stable orbits.
  • Energy is absorbed or emitted when changing orbits.
  • Explains atomic spectra, unlike Rutherford's model.
Real-world Application

This model helps in understanding light emission in LEDs. Our textbook uses hydrogen's line spectrum as an example.

Question 7:
Explain the distribution of electrons in different shells with the example of oxygen (atomic number 8).
Answer:
Concept Overview

Electrons fill shells in a step-wise manner, following the 2n² rule, where n is the shell number.

Process Explanation
  • Oxygen has 8 electrons: 2 in K-shell, 6 in L-shell.
  • First shell holds 2, second holds up to 8.
Real-world Application

This explains oxygen's reactivity, as it needs 2 more electrons to stabilize. Our textbook shows its role in water formation.

Question 8:
How did J.J. Thomson's cathode ray experiment lead to the discovery of electrons?
Answer:
Concept Overview

Thomson passed electric current through gases at low pressure, observing cathode rays.

Process Explanation
  • Rays were deflected by electric/magnetic fields, proving they are charged particles.
  • These particles were named electrons.
Real-world Application

This discovery led to TVs and CRT screens. Our textbook shows how it changed atomic theory.

Question 9:
Describe the Bohr's model of the atom and how it explains the stability of an atom.
Answer:
Concept Overview

Bohr proposed electrons move in fixed orbits around the nucleus, preventing energy loss.

Process Explanation
  • Electrons orbit in specific energy levels.
  • No radiation emitted in stationary orbits.
  • Energy changes occur when electrons jump levels.
Real-world Application

This model explains atomic spectra, like hydrogen's line spectrum. Our textbook links this to neon lights, where electron transitions produce colored light.

Question 10:
Compare the Thomson's plum pudding model and Rutherford's nuclear model of the atom.
Answer:
Concept Overview

Thomson's model had electrons embedded in a positive sphere, while Rutherford's introduced a dense nucleus.

Process Explanation
  • Thomson's model lacked a nucleus.
  • Rutherford's model had a central positive core.
  • Scattering experiments disproved Thomson's idea.
Real-world Application

Rutherford's model explains nuclear reactions, like in power plants. Our textbook contrasts this with Thomson's outdated concept.

Question 11:
Explain the distribution of electrons in different shells based on Bohr-Bury scheme.
Answer:
Concept Overview

Electrons fill shells in a step-wise manner, with the outermost shell holding up to 8 electrons.

Process Explanation
  • First shell (K) holds 2 electrons.
  • Subsequent shells hold 8, 18, etc.
  • Outermost shell determines reactivity.
Real-world Application

This explains why noble gases are inert. Our textbook uses sodium (2,8,1) as an example, showing its reactivity due to one valence electron.

Question 12:
What are isotopes? Give one NCERT example and a real-world application.
Answer:
Concept Overview

Isotopes are atoms of the same element with different neutron numbers but same protons.

Process Explanation
  • Same atomic number, different mass.
  • Example: Carbon-12 and Carbon-14.
Real-world Application

Carbon-14 is used in radiocarbon dating. Our textbook mentions hydrogen's isotopes (protium, deuterium) used in nuclear reactors.

Question 13:
Explain the Rutherford's alpha-particle scattering experiment and its conclusions. How did it lead to the discovery of the nucleus?
Answer:

Rutherford's alpha-particle scattering experiment was a landmark experiment that changed our understanding of atomic structure. In this experiment, alpha particles (positively charged helium nuclei) were fired at a thin gold foil. Most particles passed straight through, but some were deflected at small angles, and a very few bounced back.

Key observations and conclusions:

  • Most alpha particles passed through the foil undeflected, indicating that most of the atom is empty space.
  • Some particles were slightly deflected, suggesting the presence of a positively charged center (later called the nucleus).
  • A very few particles rebounded, indicating that the nucleus is extremely dense and small compared to the atom's size.

This experiment led to the discovery of the nucleus, which contains protons and neutrons, while electrons orbit around it. It disproved Thomson's plum pudding model and laid the foundation for the modern atomic model.

Question 14:
Describe the Bohr's model of the atom with its postulates. How does it explain the stability of an atom?
Answer:

Bohr's model of the atom was proposed to address the limitations of Rutherford's model. Its key postulates are:

  • Electrons revolve around the nucleus in fixed circular orbits (called stationary orbits) without radiating energy.
  • Only certain orbits are allowed, where the angular momentum of the electron is a multiple of h/2π (quantized).
  • Energy is emitted or absorbed when an electron jumps between orbits, corresponding to specific energy levels.

Bohr's model explains stability by stating that electrons in stationary orbits do not lose energy, preventing them from collapsing into the nucleus. This was a significant improvement over classical physics, which predicted that accelerating electrons would radiate energy and spiral into the nucleus.

Question 15:
Differentiate between isotopes and isobars with examples. How are isotopes useful in practical applications?
Answer:

Isotopes are atoms of the same element with the same atomic number but different mass numbers due to varying numbers of neutrons. Example: Carbon-12 and Carbon-14.

Isobars are atoms of different elements with the same mass number but different atomic numbers. Example: Argon-40 and Calcium-40.

Practical uses of isotopes:

  • Carbon-14 is used in radiocarbon dating to determine the age of fossils.
  • Uranium-235 is used as fuel in nuclear reactors.
  • Cobalt-60 is used in cancer treatment (radiotherapy).

Isotopes play a crucial role in medicine, industry, and archaeology due to their unique nuclear properties.

Question 16:
Describe the Bohr's model of the atom with its postulates. How does it explain the stability of an atom and the line spectrum of hydrogen?
Answer:

Bohr's model of the atom proposed that electrons move in fixed circular orbits or energy levels around the nucleus without radiating energy. Its key postulates are:

  • Electrons revolve in specific stationary orbits with fixed energy.
  • Energy is absorbed or emitted only when an electron jumps between orbits.
  • The angular momentum of electrons is quantized (mvr = nħ).

Stability: Since electrons do not lose energy in stationary orbits, the atom remains stable and does not collapse.

Line Spectrum: When an electron jumps from a higher to a lower orbit, it emits energy as light of a specific wavelength, forming the hydrogen line spectrum. Each transition corresponds to a unique spectral line, explaining the discrete nature of atomic spectra.

Question 17:
Explain the Rutherford's alpha-particle scattering experiment and its conclusions that led to the discovery of the nucleus in an atom. Also, mention the limitations of Rutherford's model.
Answer:

In the Rutherford's alpha-particle scattering experiment, a thin gold foil was bombarded with fast-moving alpha particles. Most particles passed straight through, but some were deflected at small angles, and a very few bounced back. This led to the following conclusions:

  • Most of the atom is empty space since most alpha particles passed through undeflected.
  • The deflection of some particles indicated a positively charged center, called the nucleus, where most of the atom's mass is concentrated.
  • The few particles that bounced back suggested the nucleus is extremely small and dense.

Limitations of Rutherford's model:

  • It could not explain the stability of electrons orbiting the nucleus (as per classical physics, they should lose energy and spiral into the nucleus).
  • It did not explain the arrangement of electrons in orbits or their energy levels.

This experiment was crucial in disproving the Thomson's plum pudding model and laid the foundation for the nuclear model of the atom.

Question 18:
Explain the Rutherford's alpha-particle scattering experiment and its conclusions that led to the discovery of the nucleus in an atom. Also, state the limitations of this model.
Answer:

In Rutherford's alpha-particle scattering experiment, a thin gold foil was bombarded with fast-moving alpha particles. Most particles passed straight through, but some were deflected at small angles, and a very few bounced back. This led to the following conclusions:

  • The atom has a tiny, dense, positively charged core called the nucleus, where most of the mass is concentrated.
  • Most of the atom is empty space, as most alpha particles passed through undeflected.
  • The electrons revolve around the nucleus in circular paths.

Limitations of Rutherford's model:

  • It could not explain the stability of atoms, as accelerating electrons should lose energy and spiral into the nucleus.
  • It did not explain the arrangement of electrons in orbits.
  • It failed to account for the line spectrum of elements.

This experiment was crucial as it replaced the Thomson's plum pudding model and laid the foundation for the modern atomic structure.

Question 19:
Explain the Rutherford's alpha-particle scattering experiment and its conclusions that led to the discovery of the nucleus in an atom. Also, state the limitations of Rutherford's model.
Answer:

In Rutherford's alpha-particle scattering experiment, a thin gold foil was bombarded with fast-moving alpha particles. Most particles passed straight through, but some were deflected at small angles, and a very few bounced back. This observation led to the following conclusions:

  • The atom has a tiny, dense, positively charged center called the nucleus, where most of its mass is concentrated.
  • Most of the atom is empty space, as most alpha particles passed through undeflected.
  • The electrons revolve around the nucleus in circular paths to balance the positive charge.

Limitations of Rutherford's model:

  • It could not explain the stability of atoms, as accelerating electrons (according to classical physics) should lose energy and spiral into the nucleus.
  • It did not explain the arrangement of electrons or their energy levels.

This experiment was crucial in disproving the Thomson's plum pudding model and laid the foundation for the modern atomic model.

Question 20:
Explain the Rutherford's alpha-particle scattering experiment and its conclusions. How did this experiment change the understanding of the structure of an atom?
Answer:

Rutherford's alpha-particle scattering experiment was a landmark experiment that provided deep insights into the structure of an atom. In this experiment, alpha particles were directed at a thin gold foil. Most particles passed straight through, but some were deflected at small angles, and a very few bounced back.


The observations led to the following conclusions:

  • Most of the atom is empty space, as most alpha particles passed through without deflection.
  • The nucleus is positively charged and extremely dense, as it caused strong repulsion of alpha particles.
  • The nucleus is very small compared to the size of the atom, as only a few particles were deflected.

This experiment disproved the Thomson's plum pudding model and introduced the nuclear model of the atom, where electrons orbit a central nucleus. It laid the foundation for modern atomic theory.

Question 21:
Describe the Bohr's model of the atom with a labeled diagram. How does it explain the stability of an atom and the line spectrum of hydrogen?
Answer:

Bohr's model of the atom proposed that electrons revolve around the nucleus in specific energy levels or orbits without radiating energy. Here’s a breakdown:


Key Postulates:

  • Electrons move in fixed circular orbits called stationary orbits.
  • Each orbit has a definite energy; energy is quantized.
  • Electrons can jump between orbits by absorbing or emitting energy in the form of photons.

Stability of the Atom: Electrons in stationary orbits do not lose energy, preventing them from spiraling into the nucleus, thus ensuring atomic stability.


Line Spectrum of Hydrogen: When an electron jumps from a higher to a lower energy level, it emits energy as light of a specific wavelength, producing the line spectrum. Each transition corresponds to a unique spectral line.


Diagram: (A simple labeled diagram showing nucleus at the center with concentric circles representing energy levels and arrows indicating electron transitions.)

Question 22:
Explain the Rutherford's alpha-particle scattering experiment and its conclusions. How did this experiment change the existing model of the atom?
Answer:

In Rutherford's alpha-particle scattering experiment, a thin gold foil was bombarded with fast-moving alpha particles. Most particles passed straight through, but some were deflected at small angles, and a very few bounced back.

Conclusions:

  • The atom has a tiny, dense, positively charged nucleus where most of its mass is concentrated.
  • Most of the atom is empty space, as most alpha particles passed through undeflected.
  • The electrons revolve around the nucleus in circular paths.

This experiment disproved Thomson's plum pudding model, which suggested that positive charge was spread uniformly. Instead, it led to the nuclear model of the atom, where the nucleus is central and electrons orbit around it.

Case-based Questions (4 Marks) – with Solutions (CBSE Pattern)

These 4-mark case-based questions assess analytical skills through real-life scenarios. Answers must be based on the case study provided.

Question 1:
In Rutherford's alpha-particle scattering experiment, most particles passed straight through the gold foil. What does this observation indicate about the structure of the atom?
Answer:
Case Summary

Rutherford observed most alpha particles passing undeflected through gold foil.

Scientific Principle
  • Atoms have mostly empty space.
  • Positive charge is concentrated in a tiny nucleus.
Solution Approach

Our textbook shows this proves the nucleus is small and dense. Real-world applications include understanding atomic models in chemistry.

Question 2:
An element has an atomic number of 11 and mass number of 23. Calculate the number of protons, neutrons, and electrons.
Answer:
Case Summary

Given atomic number (11) and mass number (23) of an element.

Scientific Principle
  • Protons = Atomic number
  • Neutrons = Mass number - Atomic number
  • Electrons = Protons in neutral atom
Solution Approach

We studied that sodium (Na) has 11 protons, 12 neutrons, and 11 electrons. This helps identify elements in the periodic table.

Question 3:
Why do isotopes of an element have the same chemical properties but different physical properties? Explain with an example.
Answer:
Case Summary

Isotopes show similar chemical behavior but vary physically.

Scientific Principle
  • Same electrons ⇒ same chemical properties
  • Different masses ⇒ different physical properties
Solution Approach

Our textbook shows hydrogen isotopes (protium, deuterium) react similarly but have different densities. This is used in nuclear reactors.

Question 4:
The electronic configuration of an element is 2,8,7. Predict its valency and group number in the periodic table.
Answer:
Case Summary

Given electronic configuration (2,8,7) of an unknown element.

Scientific Principle
  • Valency = 8 - valence electrons
  • Group number = valence electrons
Solution Approach

We studied chlorine (Cl) has this configuration with valency 1 and belongs to group 17. This helps predict chemical bonding behavior.

Question 5:
Why do isotopes of an element have the same chemical properties but different physical properties? Explain with an example.
Answer:
Case Summary

Isotopes differ in neutron number but share proton and electron count.

Scientific Principle

Chemical properties depend on electrons, while physical properties depend on mass (neutrons).

Solution Approach
  • Example: Hydrogen-1 and Hydrogen-2 (NCERT) react similarly but differ in density.
  • Same electrons = same chemistry, different mass = different physics.
Question 6:
In Bohr's model of the atom, electrons revolve in fixed orbits. What prevents electrons from losing energy and falling into the nucleus?
Answer:
Case Summary

Bohr proposed stable orbits where electrons don’t lose energy.

Scientific Principle

Our textbook states electrons in fixed orbits have quantized energy levels.

Solution Approach
  • Electrons only lose/gain energy when jumping orbits (NCERT example: hydrogen spectrum).
  • No energy loss in stationary orbits prevents collapse.
Question 7:
Rutherford's alpha-particle scattering experiment showed that most particles passed through a gold foil undeflected. Case Summary: A few particles were deflected at large angles.

Q1: What does this observation reveal about the structure of the atom?
Answer:
Case Summary: Most alpha particles passed straight, but some deflected.
Scientific Principle: Our textbook shows that atoms have a tiny, dense nucleus with positive charge.
Solution Approach:
  • Most space in an atom is empty (undeflected particles).
  • Deflection occurs due to repulsion from the positively charged nucleus (like in Rutherford’s model).

This explains why atoms are mostly empty with a central nucleus.

Question 8:
Bohr’s model explains electron orbits in hydrogen. Case Summary: Electrons occupy fixed energy levels without radiating energy.

Q2: How does this model differ from Rutherford’s nuclear model?
Answer:
Case Summary: Bohr proposed fixed orbits for electrons.
Scientific Principle: We studied that electrons in Bohr’s model have quantized energy levels (like NCERT’s hydrogen example).
Solution Approach:
  • Rutherford’s model lacked defined orbits; electrons could spiral into the nucleus.
  • Bohr’s model prevents this by assigning discrete energy levels (e.g., Lyman series).

This stabilizes the atom and explains atomic spectra.

Question 9:
An element has atomic number 11 and mass number 23. Case Summary: Its electron configuration is 2,8,1.

Q3: Identify the element and justify its valency based on this configuration.
Answer:
Case Summary: The element is sodium (Na).
Scientific Principle: Our textbook shows atomic number = protons (11 for Na).
Solution Approach:
  • Valency is 1 as it has 1 valence electron (loses 1 to achieve stability).
  • Example: Na forms NaCl by donating 1 electron (NCERT Activity 4.2).

This matches its position in Group 1 of the periodic table.

Question 10:
Isotopes of carbon (C-12 and C-14) have the same chemical properties. Case Summary: C-14 is used in carbon dating.

Q4: Why do isotopes show similar chemical behavior despite different mass numbers?
Answer:
Case Summary: Isotopes differ in neutrons but not protons.
Scientific Principle: We studied that chemical properties depend on electron configuration (same for isotopes).
Solution Approach:
  • Same atomic number = same electrons (e.g., C-12 and C-14 both have 6 electrons).
  • Application: C-14’s radioactivity doesn’t affect its bonding (used in dating fossils).

Thus, isotopes react identically in chemical reactions.

Question 11:

Rahul conducted an experiment where he passed electric current through a gas at low pressure and observed glowing rays. His teacher explained that these rays are streams of charged particles.

(a) Identify the particles constituting these rays and state their charge.
(b) How did J.J. Thomson use this experiment to propose his atomic model? Explain briefly.

Answer:

(a) The glowing rays observed are cathode rays, which consist of negatively charged particles called electrons.

(b) J.J. Thomson concluded that atoms contain electrons because these rays were deflected by electric and magnetic fields, indicating their negative charge. He proposed the plum pudding model, where:

  • Atoms are spheres of positive charge
  • Electrons are embedded in them like plums in a pudding
This experiment disproved the earlier idea that atoms were indivisible.

Question 12:

In a classroom activity, students were given three elements: Hydrogen (H), Helium (He), and Lithium (Li).

(a) Tabulate the number of protons, neutrons, and electrons in their atoms based on their atomic numbers and mass numbers.
(b) Why does Helium have a completely filled outermost shell while Lithium does not? Explain with reference to their electronic configurations.

Answer:

(a) The subatomic particle distribution is:

ElementProtonsNeutronsElectrons
Hydrogen (H)101
Helium (He)222
Lithium (Li)343

(b) Helium (atomic number 2) has the electronic configuration K2, meaning its first and only shell is completely filled with 2 electrons (duplet rule).

Lithium (atomic number 3) has the configuration K2 L1, where the outermost L-shell has only 1 electron, making it unstable. It tends to lose this electron to achieve stability.

Question 13:
Rahul conducted an experiment where he passed electric current through a gas at low pressure and observed glowing rays. He concluded that these rays were streams of negatively charged particles.

Based on this information:

  • Identify the experiment and the particles observed.
  • Explain how these particles helped in understanding the structure of the atom.
Answer:

The experiment conducted by Rahul is the discharge tube experiment, and the glowing rays observed are called cathode rays, which consist of streams of electrons (negatively charged particles).

These particles helped in understanding the structure of the atom in the following ways:

  • Discovery of electrons proved that atoms are divisible and contain smaller subatomic particles.
  • It led to the rejection of Dalton's indivisible atom theory.
  • J.J. Thomson used this discovery to propose the plum pudding model, where electrons were embedded in a positively charged sphere.

This experiment was crucial in developing modern atomic models.

Question 14:
In a classroom activity, students were given three atomic models labeled A, B, and C. Model A showed electrons embedded in a positively charged sphere, Model B depicted electrons revolving in fixed orbits, and Model C had a dense nucleus with electrons in a cloud.

Answer the following:

  • Identify each atomic model (A, B, C) with their respective scientists.
  • State one limitation of Model B.
Answer:

The atomic models can be identified as follows:

  • Model A: Thomson's Plum Pudding Model (proposed by J.J. Thomson).
  • Model B: Rutherford's Nuclear Model (proposed by Ernest Rutherford).
  • Model C: Quantum Mechanical Model (modern model based on electron probability clouds).

One limitation of Model B (Rutherford's model) is that it could not explain the stability of atoms. According to classical physics, electrons revolving around the nucleus should lose energy and spiral into the nucleus, causing the atom to collapse, which does not happen in reality.

This limitation was later addressed by Bohr's model, which introduced quantized energy levels.

Question 15:
Rahul conducted an experiment where he passed electric current through a gas at low pressure and observed glowing rays. These rays traveled from the cathode to the anode.

(a) Name the rays observed and state their nature.
(b) What conclusion did J.J. Thomson draw from this experiment about the structure of an atom?

Answer:

(a) The rays observed are called cathode rays. These rays are streams of negatively charged particles called electrons.
(b) J.J. Thomson concluded that atoms contain electrons, which are much smaller and lighter than the atom itself. This led to the discovery that atoms are divisible and have a subatomic structure. He proposed the plum pudding model, where electrons are embedded in a positively charged sphere.

Question 16:
In a classroom activity, students were asked to compare the atomic models of Rutherford and Bohr.

(a) State one key difference between their models.
(b) How did Bohr's model address the limitation of Rutherford's model regarding electron stability?

Answer:

(a) The key difference is that Rutherford's model suggested electrons move in random orbits around the nucleus, while Bohr's model proposed that electrons move in fixed energy levels or shells.
(b) Bohr's model addressed the stability issue by stating that electrons revolve in specific orbits without radiating energy. Energy is emitted or absorbed only when electrons jump between these orbits, preventing them from collapsing into the nucleus.

Question 17:
Rahul conducted an experiment where he passed electric current through a gas at low pressure and observed glowing rays. His teacher explained that these are cathode rays. Based on this:

(a) Identify the subatomic particle discovered from this experiment.
(b) State two properties of these particles.
(c) How did this experiment change the understanding of atomic structure?
Answer:

(a) The subatomic particle discovered from this experiment is the electron.

(b) Two properties of electrons are:

  • They are negatively charged particles.
  • They have a very small mass compared to protons and neutrons.

(c) This experiment led to the conclusion that atoms are divisible and contain smaller particles. It disproved the earlier idea that atoms are indivisible, paving the way for the discovery of the plum pudding model and later the nuclear model of the atom.

Question 18:
In a classroom activity, students were given cards with symbols of elements along with their atomic numbers and mass numbers. For example, one card had:

Element: Oxygen (O)
Atomic Number: 8
Mass Number: 16

(a) Calculate the number of protons, neutrons, and electrons in an oxygen atom.
(b) Draw the Bohr’s model of the oxygen atom.
(c) Why is the number of electrons equal to the number of protons in a neutral atom?
Answer:

(a) For oxygen (O):
Number of protons = Atomic Number = 8
Number of neutrons = Mass Number - Atomic Number = 16 - 8 = 8
Number of electrons = Number of protons = 8 (since the atom is neutral).

(b) Bohr’s model for oxygen (O):

  • Nucleus at the center with 8 protons and 8 neutrons.
  • Two electron shells: first shell (K) with 2 electrons, second shell (L) with 6 electrons.

(c) In a neutral atom, the number of electrons equals the number of protons because the positive charge of protons balances the negative charge of electrons, ensuring the atom has no net charge.

Question 19:
Rahul conducted an experiment where he passed electric current through a gas at low pressure and observed glowing rays. His teacher explained that these are cathode rays. Based on this:
(a) Identify the subatomic particle discovered from this experiment.
(b) State two properties of these rays observed in the experiment.
Answer:

(a) The subatomic particle discovered from this experiment is the electron, which was identified by J.J. Thomson.

(b) Two properties of cathode rays observed in the experiment are:

  • They travel in straight lines from the cathode, as seen by the glow.
  • They are negatively charged, as they were deflected towards the positive plate in an electric field.
Additionally, these rays are made up of electrons, which are fundamental particles of an atom.

Question 20:
In a classroom activity, students were given models of atoms with different numbers of protons, neutrons, and electrons. One model had 6 protons, 6 neutrons, and 6 electrons. Another had 6 protons, 8 neutrons, and 6 electrons.
(a) Name the elements represented by these models.
(b) How are these models related to each other? Explain.
Answer:

(a) Both models represent the element Carbon since the atomic number (number of protons) is 6, which is unique to Carbon.

(b) These models are isotopes of Carbon.
The first model represents Carbon-12 (6 protons + 6 neutrons).
The second model represents Carbon-14 (6 protons + 8 neutrons).
Isotopes have the same number of protons (and electrons) but different numbers of neutrons, leading to variations in atomic mass.

Question 21:
Rahul was studying the structure of the atom and came across the concept of valence electrons. He wondered why elements in the same group of the periodic table exhibit similar chemical properties. Explain this phenomenon with reference to the electronic configuration of elements, taking examples from Group 1 (Alkali Metals).
Answer:

Elements in the same group of the periodic table have similar chemical properties because they have the same number of valence electrons in their outermost shell.

For example, all Group 1 elements (Alkali Metals) like Lithium (Li), Sodium (Na), and Potassium (K) have 1 valence electron in their outermost shell.

This similarity in electronic configuration leads to identical chemical behavior, such as:

  • High reactivity with water
  • Formation of +1 ions
  • Similar bonding patterns
Thus, the arrangement of electrons, particularly valence electrons, determines the chemical properties of elements.

Question 22:
During an experiment, Priya observed that a stream of alpha particles passed through a gold foil, but some were deflected at large angles. Explain this observation based on Rutherford's atomic model, highlighting the conclusions drawn about the structure of the atom.
Answer:

Rutherford's gold foil experiment led to key discoveries about the structure of the atom. The observations and conclusions are:

Observation 1: Most alpha particles passed straight through the foil, indicating that atoms are mostly empty space.

Observation 2: Some alpha particles were deflected at large angles, suggesting a concentrated positive charge in a tiny region (later called the nucleus).

Conclusion:

  • The atom has a small, dense, positively charged nucleus at its center.
  • Electrons revolve around the nucleus in orbits.
  • Most of the atom's volume is empty space.
This experiment disproved Thomson's plum pudding model and laid the foundation for the modern atomic model.

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