NCERT Class 9 Science Journey Inside the Atom Question Answers
Short Intro
This chapter explains the fascinating journey of discovering the structure of the atom. Students learn about atomic models proposed by Dalton, Thomson, Rutherford, and Bohr along with concepts such as atomic number, mass number, isotopes, isobars, electronic configuration, and valency.
Quick Information Box
| Topic | Details |
|---|---|
| Chapter Name | Journey Inside the Atom |
| Class | Grade 9 |
| Subject | Science |
| Main Concepts | Atomic Models, Electrons, Protons, Neutrons |
| Important Scientist | Rutherford, Bohr, Chadwick |
| Key Formula | Mass Number = Protons + Neutrons |
Concepts Used (Topics Covered)
- Dalton’s Atomic Theory
- Thomson’s Plum Pudding Model
- Rutherford’s Gold Foil Experiment
- Bohr’s Atomic Model
- Discovery of Electron, Proton and Neutron
- Atomic Number and Mass Number
- Electronic Configuration
- Valency
- Isotopes and Isobars
- Average Atomic Mass
Important Formulas
- Atomic Number
Z = Number of Protons - Mass Number
A = Number of Protons + Number of Neutrons - Number of Neutrons
Neutrons = Mass Number − Atomic Number - Maximum Electrons in Shell
2n² - Average Atomic Mass
Average Mass = Σ(Isotopic Mass × Abundance)/100
Questions & Step-by-step Solutions with Explanation
Q1. Rutherford Gold Foil Experiment
(i) The experiment clearly showed the existence of neutrons in the nucleus.
Solution:
Neutrons were discovered later by James Chadwick.
Answer:
Incorrect
(ii) The results disproved the plum pudding model and led to the idea of nucleus.
Solution:
Large deflections proved positive charge is concentrated in nucleus.
Answer:
Correct
(iii) Large deflection of alpha particles showed most mass is concentrated in tiny nucleus.
Solution:
Only a dense nucleus can strongly deflect alpha particles.
Answer:
Correct
(iv) Deflection showed electrons move around nucleus.
Solution:
Gold foil experiment mainly explained nucleus, not electron motion.
Answer:
Incorrect
Q2. Bohr’s Atomic Model
(i) Electrons lose energy in fixed orbits.
Answer:
Incorrect
Explanation:
Bohr said electrons do not lose energy in fixed orbits.
(ii) Electrons can exist anywhere around nucleus.
Answer:
Incorrect
Explanation:
Electrons revolve only in fixed energy levels.
(iii) Electrons revolve in fixed energy orbits without losing energy.
Answer:
Correct
(iv) Electrons can exist between energy levels.
Answer:
Incorrect
Explanation:
Electrons cannot stay between shells.
Q3. Relation Between X, Y and Z
| Species | Protons | Neutrons |
|---|---|---|
| X | 18 | 19 |
| Y | 17 | 18 |
| Z | 17 | 20 |
(i) Relation between Y and Z
Atomic numbers same but neutrons different.
Answer:
Y and Z are isotopes.
(ii) Relation between Z and X
Mass numbers:
- Z = 17 + 20 = 37
- X = 18 + 19 = 37
Same mass number but different atomic number.
Answer:
Z and X are isobars.
Q4. Rutherford’s Conclusion
Solution:
Rutherford concluded:
- Positive charge is concentrated in a tiny nucleus.
- Most atom space is empty.
- Nucleus is dense and heavy.
Q5. Chronological Order of Atomic Models
Correct Order:
- Dalton’s Model
- Thomson’s Model
- Rutherford’s Model
- Bohr’s Model
Q6. Why Electrons Do Not Fly Away
Solution:
Electrons are negatively charged and nucleus is positively charged.
Electrostatic force of attraction keeps electrons around nucleus.
Q7. Assertion and Reason
Assertion:
Discovery of subatomic particles helped understand atomic structure.
Reason:
Number of electrons equals number of protons.
Answer:
(ii) Both are true, but R is not correct explanation of A.
Q8. Magnesium Atom
Given:
Atomic number = 12
Mass number = 24
(i) Protons
= 12
(ii) Neutrons
= 24 − 12 = 12
(iii) Electrons
= 12
Electronic Configuration
2, 8, 2
Q9. Information About Elements
(a)
Electronic configuration = 2,1
- Element = Lithium
- Symbol = Li
- Electrons = 3
- Valence electrons = 1
- Valency = 1
- Protons = 3
- Atomic number = 3
(b)
Electronic configuration = 2,8
- Element = Neon
- Symbol = Ne
- Electrons = 10
- Valency = 0
- Protons = 10
- Atomic number = 10
(c)
Electronic configuration = 2,8,1
- Element = Sodium
- Symbol = Na
- Valency = 1
(d)
Electronic configuration = 2,8,7
- Element = Chlorine
- Symbol = Cl
- Valency = 1
Q10. Rutherford vs Bohr Model
Rutherford’s Limitation:
Electrons revolving around nucleus should lose energy and collapse.
Bohr’s Explanation:
Electrons move in fixed energy levels without losing energy.
Q11. Atom ⁷⁰X with 31 Electrons
Solution:
Atomic number = 31
Neutrons = 70 − 31
= 39
Answer:
39 neutrons
Q12. Atom with 79 Protons and Mass Number 197
(i) Neutrons
= 197 − 79
= 118
(ii) Electrons
= 79
Q13. Complete the Table
| Atomic Number | Mass Number | Neutrons | Protons | Electrons | Element |
|---|---|---|---|---|---|
| 5 | 11 | 6 | 5 | 5 | Boron |
| 7 | 14 | 7 | 7 | 7 | Nitrogen |
| 12 | 24 | 12 | 12 | 12 | Magnesium |
| 15 | 31 | 16 | 15 | 15 | Phosphorus |
| 1 | 1 | 0 | 1 | 1 | Hydrogen |
Q14. Element X
Given:
Mass number = 35
Neutrons = 18
(i) Protons
= 35 − 18 = 17
Electrons
= 17
(ii) Atomic Number
17
(iii) Element
Chlorine
(iv) Electronic Configuration
2, 8, 7
(v) Valence Electrons
7
(vi) New Mass Number
35 + 2 = 37
(vii) Relation with New Atom
They are isotopes.
Q15. Hypothetical Heavy Electrons
(i) Atomic Number
No change
(ii) Atomic Mass
Increases
(iii) Mass Number
No change
(iv) Overall Charge
Neutral
Common Mistakes
- Confusing atomic number with mass number
- Forgetting neutrons calculation
- Writing incorrect shell capacities
- Mixing isotopes and isobars
- Incorrect electronic configuration
Exam Tips
- Learn all atomic models in sequence.
- Practice electron distribution regularly.
- Remember formulas for atomic number and mass number.
- Revise isotopes and isobars carefully.
- Draw neat atomic diagrams in exams.
Practice MCQs
1. Who discovered the nucleus?
A. Dalton
B. Rutherford
C. Bohr
D. Chadwick
Answer:
B. Rutherford
2. Maximum electrons in K-shell:
A. 8
B. 18
C. 2
D. 32
Answer:
C. 2
3. Atomic number equals:
A. Neutrons
B. Electrons + Neutrons
C. Protons
D. Mass number
Answer:
C. Protons
4. Isotopes have:
A. Same mass number
B. Same atomic number
C. Different electrons only
D. Same neutrons
Answer:
B. Same atomic number
5. Scientist who discovered neutron:
A. Bohr
B. Rutherford
C. Chadwick
D. Thomson
Answer:
C. Chadwick
FAQ Section
Q1. What is atomic number?
Atomic number is the number of protons in an atom.
Q2. What is mass number?
Mass number is total number of protons and neutrons.
Q3. What are isotopes?
Atoms with same atomic number but different mass numbers.
Q4. What are isobars?
Atoms with same mass number but different atomic numbers.
Q5. Why are noble gases stable?
Because they have complete outermost electron shells.
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