Welcome to our complete guide on Class 9 Science Chapter 4: Structure of the Atom! If you’re a Class 9 student aiming to excel in your 2025 CBSE exams, a parent assisting your child, or a teacher seeking detailed resources, this blog post is your one-stop solution. We’ll cover the in-text questions and answers, exercise solutions, and detailed notes for Chapter 4, simplifying the complex world of atomic structure. By the end, you’ll master concepts like electrons, protons, neutrons, and atomic models—key to scoring high and building a strong Chemistry foundation!
Chapter 4 takes you inside the atom, exploring its particles and how scientists unraveled its structure over time. From Dalton to Bohr, this chapter is packed with fascinating discoveries. Let’s dive into the Class 9 Science Chapter 4 questions, answers, and notes!
Why Study Class 9 Science Chapter 4?
Before we begin, let’s understand the importance of this chapter. The structure of the atom is the cornerstone of Chemistry, explaining how elements differ and react. It’s a high-weightage topic in the CBSE Class 9 Science syllabus (around 20-25 marks) and a stepping stone for advanced topics in higher classes. Mastering this in 2025 will boost your exam performance and prepare you for competitive exams like NTSE or Olympiads.
Let’s break this down into three sections: Notes, In-Text Questions and Answers, and Exercise Solutions.
Class 9 Science Chapter 4 Notes
1. Early Atomic Models
- Dalton’s Theory: Atoms are indivisible, solid spheres (1803).
- Thomson’s Model (1904): Atom as a “plum pudding”—positive mass with embedded electrons.
2. Discovery of Subatomic Particles
- Electrons: Negatively charged, discovered by J.J. Thomson via cathode ray experiments. Mass ≈ 1/1837 of hydrogen atom.
- Protons: Positively charged, discovered by E. Goldstein in canal ray experiments. Mass ≈ 1 u.
- Neutrons: Neutral, discovered by James Chadwick. Mass ≈ 1 u, slightly more than proton.
3. Rutherford’s Atomic Model (1911)
- Experiment: Alpha particles scattered by a gold foil.
- Conclusions:
- Atom has a tiny, dense, positive nucleus.
- Electrons orbit the nucleus.
- Most of the atom is empty space.
- Drawback: Couldn’t explain electron stability.
4. Bohr’s Atomic Model (1913)
- Electrons move in fixed orbits (energy levels) around the nucleus.
- Electrons don’t radiate energy while in these orbits.
- Energy is emitted/absorbed when electrons jump between levels.
5. Atomic Structure Terms
- Atomic Number (Z): Number of protons in the nucleus. Defines the element (e.g., H = 1, O = 8).
- Mass Number (A): Sum of protons and neutrons (A = Z + N).
- Isotopes: Atoms of the same element with different mass numbers (e.g., C-12, C-14).
- Electronic Configuration: Arrangement of electrons in shells (e.g., Na: 2, 8, 1).
6. Valency
- The combining capacity of an element, determined by electrons in the outermost shell (valence electrons).
- Examples: Na (valency 1), O (valency 2).
7. Key Facts
- Nucleus: Contains protons and neutrons, very small but holds most mass.
- Electron Shells: K (2), L (8), M (18), N (32) electrons max.
These notes summarize the chapter—use them as a quick reference!
In-Text Questions and Answers: Page 47
Question 1: What are canal rays?
Answer:
Canal rays are positively charged rays discovered by E. Goldstein in a gas discharge tube. They consist of positive ions formed when electrons knock off from gas atoms, leaving them positively charged. These rays led to the discovery of protons.
Question 2: If an atom contains one electron and one proton, will it carry any charge or not?
Answer:
No, the atom will be neutral. One electron (-1 charge) and one proton (+1 charge) cancel each other out. Example: Hydrogen-1 (protium) has 1 proton and 1 electron, making it neutral.
In-Text Questions and Answers: Page 50
Question 1: On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Answer:
Thomson’s “plum pudding” model describes the atom as a sphere of positive charge with electrons embedded like plums. The total positive charge equals the total negative charge of the electrons, making the atom electrically neutral overall.
Question 2: On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Answer:
According to Rutherford’s model, the proton (positively charged) is present in the nucleus. Later, neutrons were also found there, but Rutherford’s experiment identified the nucleus as positive, pointing to protons.
In-Text Questions and Answers: Page 52
Question 1: Name the three subatomic particles of an atom.
Answer:
The three subatomic particles are:
- Electrons: Negatively charged, orbit the nucleus.
- Protons: Positively charged, in the nucleus.
- Neutrons: Neutral, in the nucleus.
Question 2: Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Answer:
Mass number (A) = Protons + Neutrons.
For helium: A = 4 u, Protons = 2.
Neutrons = A – Protons = 4 – 2 = 2 neutrons.
In-Text Questions and Answers: Page 53
Question 1: Write the distribution of electrons in carbon and sodium atoms.
Answer:
- Carbon (Z = 6): 6 electrons. Distribution: K (2), L (4).
- Sodium (Z = 11): 11 electrons. Distribution: K (2), L (8), M (1).
Question 2: If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer:
- K shell holds 2 electrons.
- L shell holds 8 electrons.
- Total = 2 + 8 = 10 electrons.
This corresponds to Neon (Z = 10).
Exercise Questions and Answers: Page 57
Question 1: Compare the properties of electrons, protons, and neutrons.
Answer:
| Particle | Charge | Mass (u) | Location |
|---|---|---|---|
| Electron | Negative (-1) | ~1/1837 | Orbits nucleus |
| Proton | Positive (+1) | ~1 | In nucleus |
| Neutron | Neutral (0) | ~1 | In nucleus |
Question 2: What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?
Answer:
Rutherford used gold because it’s malleable and can be made into thin foils. Using another metal (e.g., silver or aluminum) would give similar results—most α-particles pass through, some deflect, a few bounce back—since all atoms have a dense nucleus. The exact scattering pattern might vary slightly due to differences in nuclear size or density, but the conclusion (nucleus exists) remains the same.
Question 3: Write the electronic configuration of any one pair of isotopes and isobars.
Answer:
- Isotopes: Carbon-12 (¹²C) and Carbon-14 (¹⁴C).
- ¹²C: Z = 6, electrons = 2, 4.
- ¹⁴C: Z = 6, electrons = 2, 4.
(Same Z, different neutrons: 6 vs. 8).
- Isobars: Carbon-14 (¹⁴C) and Nitrogen-14 (¹⁴N).
- ¹⁴C: Z = 6, electrons = 2, 4.
- ¹⁴N: Z = 7, electrons = 2, 5.
(Same A, different Z).
Question 4: For the following elements, write the number of protons, neutrons, and electrons:
(a) ¹²C
(b) ³⁵Cl
Answer:
(a) ¹²C:
- Protons = Z = 6.
- Mass number = 12, Neutrons = 12 – 6 = 6.
- Electrons = 6 (neutral atom).
(b) ³⁵Cl:
- Protons = Z = 17.
- Mass number = 35, Neutrons = 35 – 17 = 18.
- Electrons = 17 (neutral atom).
Question 5: If Z = 3, what would be the valency of the element? Also, name the element.
Answer:
- Z = 3 is Lithium (Li).
- Electronic configuration: 2, 1.
- Valency = 1 (1 electron in outer shell to lose).
- Element: Lithium.
Tips to Master Chapter 4
- Learn Models: Understand Thomson, Rutherford, and Bohr’s contributions.
- Practice Configurations: Write electron distributions for Z = 1 to 20.
- Memorize Terms: Focus on atomic number, mass number, isotopes, and valency.
- Visualize: Draw atomic structures to grasp nucleus and orbits.
Class 9 Science Chapter 4: Structure of the Atom unveils the mysteries of atomic composition, from early models to modern understanding. With these notes, questions, and solutions, you’re ready to shine in your 2025 CBSE exams. This chapter blends history, theory, and calculations—making it both intriguing and scoring.
Explore NCERT solutions and sample papers for extra practice. Got doubts? Leave them in the comments—we’re here to assist! Keep learning and stay curious!
For all Class 9 Science Chapters: Notes and Question Answers. Click Here.