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Political Science NCERT Solutions Class 9 Science Chapter 4 – Structure of the Atom
Class 9 Science
Last Updated: 15 April 2026
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Important Points
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Practice MCQ Questions (26 questions)
Q1. Who discovered the electron?
✓ Correct Answer: D
Q2. What model of the atom was proposed by J.J. Thomson?
✓ Correct Answer: B
Q3. What was the name of the positively charged particles discovered by E. Goldstein?
✓ Correct Answer: D
Q4. What experiment led to the discovery of the atomic nucleus?
✓ Correct Answer: D
Q5. Who discovered the neutron?
✓ Correct Answer: B
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Frequently Asked Questions (FAQ)
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Tough Words (Glossary)
Sub-atomic particle
Meaning: A particle smaller than an atom (electron, proton, neutron)
अर्थ: परमाणु से छोटा कण (जैसे इलेक्ट्रॉन, प्रोटॉन, न्यूट्रॉन)
"Electrons are sub-atomic particles."
Electron
Meaning: Negatively charged sub-atomic particle
अर्थ: ऋण आवेशित उप-परमाणुक कण
"An electron is denoted by e−."
Proton
Meaning: Positively charged sub-atomic particle in the nucleus
अर्थ: नाभिक में स्थित धन आवेशित कण
"A proton is denoted by p+."
Neutron
Meaning: Neutral sub-atomic particle in the nucleus
अर्थ: नाभिक में स्थित आवेश रहित कण
"Neutrons carry no charge."
Canal rays
Meaning: Positively charged rays discovered in gas discharge
अर्थ: गैस निर्वहन में खोजी गई धन आवेशित किरणें
"Goldstein discovered canal rays."
Alpha particle
Meaning: Doubly charged helium ion (He2+)
अर्थ: दुगुना धन आवेशित हीलियम आयन (He2+)
"Alpha particles were used in the gold foil experiment."
Gold foil experiment
Meaning: Rutherford’s experiment scattering alpha particles by gold foil
अर्थ: रदरफोर्ड का सोने की पन्नी से अल्फा कणों का प्रकीर्णन प्रयोग
"The gold foil experiment revealed the nucleus."
Scattering
Meaning: Deflection of particles from their path
अर्थ: कणों का पथ से विचलन (प्रकीर्णन)
"Some alpha particles showed scattering."
Nucleus
Meaning: Positively charged central part of an atom
अर्थ: परमाणु का धन आवेशित केंद्रीय भाग (नाभिक)
"Nearly all atomic mass is in the nucleus."
Electrically neutral
Meaning: Having equal positive and negative charges
अर्थ: विद्युततः उदासीन (धन व ऋण आवेश बराबर)
"An atom as a whole is electrically neutral."
Static electricity
Meaning: Electric charge at rest
अर्थ: स्थिर विद्युत
"Combing dry hair shows static electricity."
Model of atom
Meaning: A proposed arrangement of particles in an atom
अर्थ: परमाणु में कणों की प्रस्तावित रचना (मॉडल)
"Thomson proposed an early model of atom."
Thomson’s model
Meaning: Electrons embedded in a positive sphere (plum pudding model)
अर्थ: धनात्मक गोले में जड़े इलेक्ट्रॉन (प्लम पुडिंग मॉडल)
"Thomson’s model could not explain later results."
Rutherford’s model
Meaning: Electrons revolve around a small dense nucleus
अर्थ: छोटे सघन नाभिक के चारों ओर घूमते इलेक्ट्रॉन
"Rutherford proposed the nuclear model."
Bohr’s model
Meaning: Electrons in fixed energy orbits without radiating energy
अर्थ: निश्चित ऊर्जा कक्षाओं में ऊर्जा का विकिरण किए बिना घूमते इलेक्ट्रॉन
"Bohr introduced discrete orbits."
Discrete orbit
Meaning: Specific allowed path with fixed energy
अर्थ: निश्चित ऊर्जा वाली अनुमति प्राप्त कक्षा
"Electrons move in discrete orbits."
Radiate
Meaning: To emit energy as waves or particles
अर्थ: विकिरण करना
"Accelerating charges radiate energy."
Deflection
Meaning: Change in direction of motion
अर्थ: दिशा में परिवर्तन (विक्षेप)
"Few alpha particles showed large deflection."
Rebound
Meaning: To bounce back after striking
अर्थ: टकराकर लौट आना
"Rare alpha particles rebounded from the foil."
Incredible
Meaning: Difficult to believe; astonishing
अर्थ: अविश्वसनीय
"Rutherford called the result almost incredible."
Centripetal acceleration
Meaning: Acceleration towards the centre of circular motion
अर्थ: वृत्तीय गति में केंद्राभिमुख त्वरण
"Circular motion involves centripetal acceleration."
Stability
Meaning: Ability to remain unchanged or not fall apart
अर्थ: स्थिरता
"Bohr’s model explained atomic stability."
Shell (energy level)
Meaning: A region around the nucleus where electrons are found
अर्थ: नाभिक के चारों ओर इलेक्ट्रॉनों का ऊर्जा स्तर (खोल)
"Electrons occupy different shells."
Bohr–Bury scheme
Meaning: Rule for electron distribution among shells
अर्थ: खोलों में इलेक्ट्रॉनों के वितरण का नियम (बोर-बरी)
"Use the Bohr–Bury scheme to fill electrons."
2n² rule
Meaning: Maximum electrons in a shell equals 2n²
अर्थ: किसी खोल में अधिकतम इलेक्ट्रॉन = 2n²
"The L-shell can hold 8 electrons by 2n² rule."
Valence electrons
Meaning: Electrons in the outermost shell
अर्थ: बाह्यतम खोल के इलेक्ट्रॉन (संयोजी इलेक्ट्रॉन)
"Chemical bonding depends on valence electrons."
Valency
Meaning: Combining capacity based on electrons lost, gained or shared
अर्थ: इलेक्ट्रॉन खोने, पाने या साझा करने पर आधारित संयोजकता
"Sodium has a valency of one."
Octet
Meaning: A stable arrangement of eight electrons in the outer shell
अर्थ: बाह्य खोल में आठ इलेक्ट्रॉनों की स्थिर व्यवस्था
"Atoms react to achieve an octet."
Inert (noble) gases
Meaning: Elements with complete outer shells, low reactivity
अर्थ: पूर्ण बाह्य खोल वाले, कम अभिक्रियाशील गैसें
"Neon is an inert gas."
Atomic number (Z)
Meaning: Number of protons in the nucleus
अर्थ: नाभिक में प्रोटॉनों की संख्या (परमाणु क्रमांक)
"Carbon has atomic number 6."
Mass number (A)
Meaning: Total number of protons and neutrons
अर्थ: प्रोटॉन + न्यूट्रॉन की कुल संख्या (द्रव्यमान संख्या)
"Aluminium has mass number 27."
Nucleons
Meaning: Protons and neutrons in the nucleus
अर्थ: नाभिक के कण—प्रोटॉन व न्यूट्रॉन
"Mass of an atom is due to nucleons."
Isotopes
Meaning: Atoms of same element with same Z but different A
अर्थ: एक ही तत्व के समान Z पर भिन्न A वाले परमाणु (समस्थानिक)
"Hydrogen has three isotopes."
Isobars
Meaning: Atoms of different elements with same mass number
अर्थ: भिन्न तत्वों के समान द्रव्यमान संख्या वाले परमाणु (समभारक)
"Calcium-40 and argon-40 are isobars."
Average atomic mass
Meaning: Weighted average of isotopic masses
अर्थ: समस्थानिक द्रव्यमानों का भारित औसत
"Chlorine’s average atomic mass is 35.5 u."
Notation (AZX)
Meaning: Symbol showing A (mass), Z (atomic number), and element
अर्थ: A (द्रव्यमान), Z (क्रमांक) व तत्व दिखाने वाला संकेतन
"Write ¹²₆C for carbon-12."
Charge
Meaning: Property causing electrical attraction or repulsion
अर्थ: आवेश
"Electrons have negative charge."
Magnitude
Meaning: Size or amount of a quantity
अर्थ: परिमाण
"Proton and electron have equal charge magnitude."
Neutral atom
Meaning: Atom with equal numbers of protons and electrons
अर्थ: प्रोटॉन व इलेक्ट्रॉन बराबर वाला उदासीन परमाणु
"A neutral atom has no net charge."
Gas discharge tube
Meaning: Apparatus to study conduction in gases
अर्थ: गैस निर्वहन नलिका
"Electrons were studied using gas discharge tubes."
Cavendish Laboratory
Meaning: Famous research lab at Cambridge University
अर्थ: कैवेंडिश प्रयोगशाला (कैम्ब्रिज)
"Thomson directed the Cavendish Laboratory."
Indestructible (historical)
Meaning: Once thought impossible to destroy or break
अर्थ: अविनाशी (ऐतिहासिक धारणा)
"Dalton considered atoms indestructible."
Experimental evidence
Meaning: Proof obtained from experiments
अर्थ: प्रायोगिक प्रमाण
"Models changed with new experimental evidence."
Energy level
Meaning: Quantised energy state of an electron
अर्थ: इलेक्ट्रॉन की मात्रित ऊर्जा अवस्था
"Electrons jump between energy levels."
Outer shell
Meaning: The outermost electron shell
अर्थ: बाह्यतम खोल
"Reactivity depends on the outer shell."
Gain/Lose/Share (electrons)
Meaning: Ways atoms attain octet
अर्थ: इलेक्ट्रॉन पाना/खोना/साझा करना
"Sodium loses one electron to form Na+."
Combining capacity
Meaning: Ability of an atom to combine (valency)
अर्थ: संयोजन क्षमता (संयोजकता)
"Chlorine’s combining capacity is one."
Distributed (electrons)
Meaning: Arranged among shells
अर्थ: इलेक्ट्रॉनों का वितरण
"Electrons are distributed as 2,8,8…"
Weighted average
Meaning: Average that accounts for different proportions (weights)
अर्थ: भारित औसत
"Average atomic mass is a weighted average of isotopes."
Pure substance
Meaning: Material with uniform and definite composition
अर्थ: शुद्ध पदार्थ
"Each isotope is a pure substance."