Journey Inside the Atom — Class 9 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 9 Science chapter "Journey Inside the Atom" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 9 Science chapter "Journey Inside the Atom" — 8 important questions with detailed answers for CBSE board e…
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NCERT Solutions for Class 9 Science — Journey Inside the Atom. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Journey Inside the Atom — All 8 Questions Solved
Q1. What are the three subatomic particles and their charges?
The three subatomic particles are:
(1) Electron — negatively charged (charge = -1), mass = 1/1836 amu, found in orbitals around nucleus
(2) Proton — positively charged (charge = +1), mass = 1 amu, present in nucleus
(3) Neutron — neutral (charge = 0), mass ≈ 1 amu, present in nucleus
Q2. State and explain Thomson's model of atom.
Thomson's model (plum pudding model):
Atom is a sphere of positive charge with electrons embedded in it like raisins in a pudding.
Characteristics: (1) Positive charge uniformly distributed throughout atom
(2) Electrons scattered throughout in this positive charge
(3) Atom is electrically neutral
Limitations: Could not explain scattering of α-particles; disproved by Rutherford's experiments.
Q3. What are the main features of Rutherford's model of atom?
Rutherford's nuclear model:
(1) Atom has a small, dense, positively charged nucleus at center
(2) Electrons revolve around nucleus in circular paths
(3) Most of atom's mass is concentrated in nucleus
(4) Nucleus contains protons; electrons in orbits
(5) Atom is mostly empty space
Limitation: Could not explain stability of electrons; they should spiral into nucleus.
Q4. Explain Bohr's model of atom. Why is it better than Rutherford's?
Bohr's model:
(1) Electrons exist in fixed orbits/shells (n = 1, 2, 3...) at specific distances from nucleus
(2) Electrons in a shell have definite energy; transition between shells requires energy
(3) Orbits are labeled K, L, M, N (n = 1, 2, 3, 4...)
(4) Maximum electrons in shell = 2n²
Advantages over Rutherford: (1) Explains why electrons don't spiral into nucleus
(2) Explains discrete energy levels and spectral lines
(3) Applies to hydrogen and hydrogen-like atoms
Q5. Define atomic number and mass number. How are they related?
Atomic number (Z) = Number of protons in nucleus = Number of electrons in neutral atom
Mass number (A) = Total number of protons + neutrons = Z + N
where N = number of neutrons
Example: ₁₆³²S has Z = 16 (16 protons), A = 32, so N = 32 - 16 = 16 neutrons
Atomic number determines element identity; mass number determines isotope.
Q6. What is valency? How does electron configuration determine valency?
Valency = Number of electrons lost, gained, or shared by atom to achieve stable octet (8 electrons in outermost shell) or duplet (2 electrons for H, He).
Rules: (1) If valence electrons < 4: valency = number of valence electrons
(2) If valence electrons ≥ 4: valency = 8 - number of valence electrons
Example: Chlor (17e⁻): 2, 8, 7 → 7 valence electrons → valency = 8 - 7 = 1
Oxygen (8e⁻): 2, 6 → 6 valence electrons → valency = 8 - 6 = 2
Q7. Define isotopes and give two examples. How do isotopes differ?
Isotopes = Atoms of same element having same atomic number but different mass numbers (different number of neutrons).
Example 1: ₆¹²C and ₆¹⁴C (Carbon-12 and Carbon-14)
Example 2: ₁₇³⁵Cl and ₁₇³⁷Cl (Chlorine-35 and Chlorine-37)
Differences: (1) Same atomic number Z (same chemical properties)
(2) Different mass numbers A (different physical properties like density, melting point)
(3) Different number of neutrons (N differs)
(4) Some isotopes are radioactive (C-14, Cl-37)
Q8. An atom has 8 protons and 8 neutrons. Name the element and write its atomic notation.
Number of protons = Atomic number = 8 → Element is Oxygen (O)
Mass number = Protons + Neutrons = 8 + 8 = 16
Atomic notation = ₈¹⁶O (or O-16)
Electron configuration = 2, 6
Valency = 8 - 6 = 2 (divalent)
Journey Inside the Atom — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Who discovered the electron through his cathode ray tube experiments?
- A. E. Goldstein
- B. J.J. Thomson
- C. James Chadwick
- D. Ernest Rutherford
Answer: B. J.J. Thomson — J.J. Thomson conducted the cathode ray tube experiments that led to the discovery of the electron.
MCQ 2. What is the nature of the electric charge on an electron?
- A. Positive
- B. Negative
- C. Neutral
- D. Variable
Answer: B. Negative — Electrons are fundamental subatomic particles that carry a negative electric charge.
MCQ 3. Which subatomic particle has no electric charge?
- A. Proton
- B. Electron
- C. Neutron
- D. Positron
Answer: C. Neutron — Neutrons are electrically neutral subatomic particles found within the nucleus of an atom.
MCQ 4. What does the atomic number (Z) of an element represent?
- A. Number of neutrons
- B. Number of protons
- C. Number of electrons
- D. Sum of protons and neutrons
Answer: B. Number of protons — The atomic number (Z) is defined as the number of protons in the nucleus of an atom and uniquely identifies an element.
MCQ 5. Which atomic model is also known as the 'plum pudding model'?
- A. Rutherford's Model
- B. Bohr's Model
- C. Thomson's Model
- D. Dalton's Model
Answer: C. Thomson's Model — J.J. Thomson proposed the plum pudding model, where electrons were embedded in a positively charged sphere, much like plums in a pudding.
MCQ 6. According to Bohr-Bury rules, what is the maximum number of electrons that can be accommodated in the K shell (first shell)?
- A. 2
- B. 8
- C. 18
- D. 32
Answer: A. 2 — The K shell (n=1) is the innermost electron shell and can hold a maximum of 2 electrons based on the 2n² rule.
MCQ 7. Atoms of the same element having the same atomic number but different mass numbers are called what?
- A. Isobars
- B. Isotopes
- C. Ions
- D. Allotropes
Answer: B. Isotopes — Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons, leading to different mass numbers.
MCQ 8. What is the dense, positively charged central part of an atom called?
- A. Orbit
- B. Shell
- C. Nucleus
- D. Electron cloud
Answer: C. Nucleus — The nucleus is the small, dense, and positively charged core of an atom, containing protons and neutrons.
MCQ 9. Rutherford's alpha-particle scattering experiment led to the conclusion that:
- A. Atoms are indivisible
- B. The entire mass of the atom is spread uniformly
- C. Most of the space in an atom is empty
- D. Electrons are embedded in a positive sphere
Answer: C. Most of the space in an atom is empty — The observation that most alpha particles passed straight through the gold foil without deflection indicated that most of the atom's volume is empty space.
MCQ 10. An atom has 11 protons, 12 neutrons, and 11 electrons. What is its atomic number and mass number?
- A. Z=11, A=11
- B. Z=11, A=12
- C. Z=11, A=23
- D. Z=12, A=23
Answer: C. Z=11, A=23 — The atomic number (Z) equals the number of protons (11). The mass number (A) is the sum of protons and neutrons (11 + 12 = 23).
MCQ 11. If an atom has an electronic configuration of 2, 8, 1, what is its valency?
- A. 1
- B. 2
- C. 7
- D. 8
Answer: A. 1 — An atom with 1 electron in its outermost shell will readily lose this electron to achieve a stable octet, giving it a valency of 1.
MCQ 12. When an atom loses an electron, it forms a:
- A. Anion
- B. Cation
- C. Isotope
- D. Molecule
Answer: B. Cation — By losing electrons, an atom acquires a net positive charge because the number of protons becomes greater than the number of electrons, forming a cation.
MCQ 13. Which isotope of uranium is commonly used as a fuel in nuclear reactors?
- A. Uranium-238
- B. Uranium-235
- C. Uranium-232
- D. Uranium-240
Answer: B. Uranium-235 — Uranium-235 is the only naturally occurring fissile isotope and is primarily used as fuel in nuclear power reactors.
MCQ 14. According to Bohr's model, electrons do not radiate energy while revolving in:
- A. Any orbit
- B. Specific discrete orbits
- C. The nucleus
- D. The electron cloud
Answer: B. Specific discrete orbits — Bohr's model postulated that electrons revolve only in certain stable, discrete orbits or shells without radiating energy.
MCQ 15. Cathode rays are composed of:
- A. Protons
- B. Electrons
- C. Neutrons
- D. Alpha particles
Answer: B. Electrons — J.J. Thomson's experiments showed that cathode rays are streams of negatively charged particles, which were identified as electrons.
MCQ 16. Which phenomenon could NOT be explained by Thomson's model of the atom?
- A. Stability of the atom
- B. Neutrality of the atom
- C. Distribution of positive and negative charges
- D. Alpha-particle scattering experiment results
Answer: D. Alpha-particle scattering experiment results — Thomson's model could not account for the large deflections of alpha particles observed in Rutherford's experiment, which contradicted its prediction of minor deflections.
MCQ 17. In Rutherford's alpha-particle scattering experiment, a very small fraction of alpha particles were deflected back. This indicated that:
- A. The positive charge is uniformly distributed
- B. The nucleus is very large
- C. Most of the atom's mass is concentrated in a tiny central region
- D. Electrons are in fixed orbits
Answer: C. Most of the atom's mass is concentrated in a tiny central region — The rare, large-angle deflections and complete rebounds suggested that the atom's positive charge and most of its mass are concentrated in a very small, dense nucleus.
MCQ 18. An atom of element X has 17 protons and 18 neutrons. It forms an ion X⁻. What is the number of electrons in the X⁻ ion?
- A. 16
- B. 17
- C. 18
- D. 19
Answer: C. 18 — The atomic number (number of protons) is 17. A neutral atom has 17 electrons. An X⁻ ion means it has gained one electron, so it will have 17 + 1 = 18 electrons.
MCQ 19. An element has its K, L, and M shells completely filled. What is its valency?
- A. 0
- B. 1
- C. 2
- D. 8
Answer: A. 0 — If all electron shells are completely filled (e.g., K=2, L=8, M=18), the atom has a stable configuration and no tendency to gain or lose electrons, resulting in a valency of 0.
MCQ 20. Which of the following atomic models correctly proposed that electrons revolve in discrete orbits?
- A. Dalton's model
- B. Thomson's model
- C. Rutherford's model
- D. Bohr's model
Answer: D. Bohr's model — Niels Bohr's model introduced the concept that electrons revolve only in specific, discrete energy levels or orbits, providing stability to the atom.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.