Class 9 Science · Chapter 9
Short answer: Class 9 Science Chapter 9 "Atomic Foundations of Matter" (purana naam: Atoms and Molecules) me laws of chemical combination, atoms-molecules-ions, chemical formula writing (valency), aur mole concept (Avogadro number) cover hota hai — yahi topics Class 10 Chemistry ki neev bante hain.
Yeh chapter — purane naam se "Atoms and Molecules", ab renamed "Atomic Foundations of Matter" (Chapter 9, Exploration textbook) — poore Class 10 Chemistry (mole concept, stoichiometry, molarity) ki neev hai. Yahan se shuruaat hoti hai: matter kaise fixed proportions me combine hoti hai (laws of chemical combination), atoms ko kaise symbol aur mass diya jaata hai, molecules aur ions kya hote hain, chemical formula kaise likhte hain, aur sabse zyada exam-heavy topic — mole concept.
Is chapter ka structure straightforward hai: pehle do laws (conservation of mass, constant proportions) samajhte hain jo Dalton ki atomic theory ki base bane. Fir atoms — symbols, atomic mass unit — cover karte hain. Uske baad molecules, atomicity, aur ions (cations/anions) aate hain. Chemical formula writing ka criss-cross method sabse zyada practice maangta hai. Aur last me molecular mass, formula unit mass, aur mole concept (Avogadro number 6.022×10²³) — jo numerically sabse zyada tricky part hai lekin ek baar formula clear ho jaaye to straightforward ban jaata hai.
Chapter 9 Summary — 5 Minute Revision
Is chapter me do foundational laws se shuruaat hoti hai — law of conservation of mass (reactants ka total mass = products ka total mass) aur law of constant proportions (ek compound me elements hamesha fixed mass-ratio me hote hain). Ye dono laws Dalton ki atomic theory ki base bane, jiske postulates atoms ki indivisibility aur unke fixed combining ratio explain karte hain.
Fir atoms ko symbols (IUPAC convention) aur atomic mass unit (u = 1/12th mass of C-12 atom) se define kiya jaata hai. Molecules — elements ke (O₂, O₃, P₄, S₈) aur compounds ke (H₂O, CO₂) — aur unki atomicity cover hoti hai, saath me ions (cations jaise Na⁺, anions jaise Cl⁻, aur polyatomic ions jaise SO₄²⁻).
Chemical formula writing valency-based criss-cross method se sikhaya jaata hai, jisme polyatomic ions pe bracket ka use zaroori hai. Last section molecular mass, formula unit mass, aur mole concept ko cover karta hai — Avogadro number (6.022×10²³) ke through moles ko mass aur number of particles se connect kiya jaata hai, jo numerically is chapter ka sabse important skill hai.
In-Text Questions — Solutions
5.3 g sodium carbonate, 6 g ethanoic acid ke saath react karta hai. Products hain: 2.2 g carbon dioxide, 0.9 g water, aur 8.2 g sodium ethanoate. Dikhao ki ye observations law of conservation of mass ke agreement me hain.
Reactants ka total mass:
5.3 g + 6 g = 11.3 g
Products ka total mass:
2.2 g + 0.9 g + 8.2 g = 11.3 g
Reactants ka total mass (11.3 g) = Products ka total mass (11.3 g). Dono equal hain, isliye ye observations law of conservation of mass ke saath agreement me hain.
Hydrogen aur oxygen 1:8 mass ratio me combine hokar water banate hain. 3 g hydrogen gas ko completely react karne ke liye kitni oxygen gas chahiye hogi?
Given ratio: H : O = 1 : 8 by mass
Mass of oxygen required = 3 g × 8 = 24 g
Atomic mass unit define karo.
Atomic mass unit (u) ek carbon-12 atom ke mass ka 1/12th hissa hota hai. Iska use isliye kiya jaata hai kyunki actual atomic mass in grams itna chota hota hai ki har baar exponential number likhna impractical ho jaata.
Atom ko naked eyes se dekhna kyun possible nahi hai?
Atom ka size itna chota hota hai (radius ~10⁻¹⁰ m order pe) ki human eye — ya even normal light microscope — ki resolving power se bhi kaafi neeche hai. Isliye individual atoms sirf specialised instruments (jaise electron microscope) se hi "dekhe" ja sakte hain, naked eye se nahi.
In compounds ke formulae likho: (i) sodium oxide (ii) aluminium chloride (iii) sodium sulphide (iv) magnesium hydroxide
(i) Na valency 1, O valency 2 → Na₂O
(ii) Al valency 3, Cl valency 1 → AlCl₃
(iii) Na valency 1, S valency 2 → Na₂S
(iv) Mg valency 2, OH⁻ valency 1 → Mg(OH)₂
In formulae se compound ke naam likho: (i) Al₂(SO₄)₃ (ii) CaCl₂ (iii) MgSO₄ (iv) Ca(OH)₂
(i) Al₂(SO₄)₃ → Aluminium sulphate
(ii) CaCl₂ → Calcium chloride
(iii) MgSO₄ → Magnesium sulphate
(iv) Ca(OH)₂ → Calcium hydroxide
"Chemical formula" term se kya matlab hai?
Chemical formula kisi compound ki symbolic representation hai jo elements ke symbols aur unke subscripts (relative number of atoms/ions) use karke us compound ke ek unit (molecule ya formula unit) ki exact composition dikhati hai.
In me kitne atoms hain: (i) H₂S molecule (ii) PO₄³⁻ ion?
(i) H₂S: 2 H atoms + 1 S atom = 3 atoms
(ii) PO₄³⁻: 1 P atom + 4 O atoms = 5 atoms
H₂O aur CO₂ ka molecular mass, aur ZnO, Na₂O, K₂CO₃ ka formula unit mass calculate karo. (Atomic mass: H=1, O=16, C=12, Zn=65, Na=23, K=39)
Molecular mass:
H₂O = (2×1) + 16 = 18 u
CO₂ = 12 + (2×16) = 44 u
Formula unit mass:
ZnO = 65 + 16 = 81 u
Na₂O = (2×23) + 16 = 62 u
K₂CO₃ = (2×39) + 12 + (3×16) = 78 + 12 + 48 = 138 u
In ka mass calculate karo: (i) 1 mole nitrogen atoms (N=14) (ii) 4 moles aluminium atoms (Al=27) (iii) 10 moles sodium sulphite Na₂SO₃
(i) 1 mole N atoms = 14 g
(ii) 4 moles Al atoms = 4 × 27 = 108 g
(iii) Molar mass Na₂SO₃ = (2×23) + 32 + (3×16) = 46 + 32 + 48 = 126 g/mol
10 moles Na₂SO₃ = 10 × 126 = 1260 g
In ko mole me convert karo: (i) 12 g oxygen gas (ii) 20 g water (iii) 22 g carbon dioxide. (Molar mass: O₂=32, H₂O=18, CO₂=44)
(i) Moles of O₂ = 12/32 = 0.375 mol
(ii) Moles of H₂O = 20/18 = 1.11 mol
(iii) Moles of CO₂ = 22/44 = 0.5 mol
Agar 1 mole carbon atoms ka mass 12 g hai, to carbon ke 1 atom ka mass grams me kitna hoga? Aur 16 g solid sulphur (S₈) me kitne molecules honge?
Mass of 1 carbon atom:
= 12 g ÷ 6.022 × 10²³ = 1.993 × 10⁻²³ g
S₈ molecules in 16 g sulphur:
Molar mass S₈ = 8 × 32 = 256 g/mol
Moles = 16/256 = 0.0625 mol
Number of molecules = 0.0625 × 6.022 × 10²³ = 3.76 × 10²² molecules
Exercise Questions — Solutions (Q1–Q11)
3.0 g carbon, 8.00 g oxygen ke saath completely react karke sirf carbon dioxide banata hai. Kitna carbon dioxide banega, aur is numerical se kaunsa law illustrate hota hai?
Step 1 — ratio check: C ka mole = 3/12 = 0.25 mol; O2 ka mole = 8/32 = 0.25 mol. C + O2 → CO2 me 1:1 mole ratio chahiye — match ho gaya, matlab reaction complete hai, kuch bhi unreacted nahi bachega.
Mass of CO₂ = mass of C + mass of O₂ = 3 g + 8 g = 11 g
Law illustrated: Law of Conservation of Mass — reactants ka total mass (11 g) products ke total mass (11 g) ke barabar hai, koi mass create ya destroy nahi hua.
Dalton ki atomic theory ka kaunsa postulate law of conservation of mass ka result hai?
Postulate: "Atoms can neither be created nor destroyed in a chemical reaction."
Reasoning: agar reaction me koi atom na banta hai na khatam hota hai, sirf rearrange hota hai, to total mass (jo atoms ki sankhya × unke mass se decide hota hai) bhi constant rahega — yahi law of conservation of mass hai.
Dalton ki atomic theory ka kaunsa postulate law of constant proportions ko explain karta hai?
Postulate: "The relative number and kinds of atoms in a given compound are constant."
Reasoning: ek compound ke fixed number of atoms of each element hote hain (jaise water me hamesha 2 H aur 1 O), aur har atom ka mass bhi fixed hota hai — isliye us compound me elements ka mass-ratio bhi hamesha constant rahega, chahe compound kahin bhi banaya jaye ya kitni bhi quantity me banaya jaye.
In elements ke IUPAC symbols likho: Aluminium, Potassium, Iron, Silver, Sodium.
Aluminium → Al
Potassium → K (Latin: Kalium)
Iron → Fe (Latin: Ferrum)
Silver → Ag (Latin: Argentum)
Sodium → Na (Latin: Natrium)
Note: element ke symbol ka pehla letter hamesha capital hota hai, doosra letter (agar ho) small — jaise Co (Cobalt) aur CO (Carbon monoxide) me confuse mat karo.
In compounds ke chemical formulae likho: (i) calcium oxide (ii) magnesium chloride (iii) copper nitrate (iv) potassium sulphate
(i) Calcium oxide: Ca valency 2, O valency 2 → equal, cancel ho jate hain → CaO
(ii) Magnesium chloride: Mg valency 2, Cl valency 1 → criss-cross se Cl ki subscript 2 → MgCl₂
(iii) Copper nitrate: Cu valency 2, NO₃⁻ valency 1 → bracket zaroori kyunki polyatomic ion ki count 1 se zyada honi hai → Cu(NO₃)₂
(iv) Potassium sulphate: K valency 1, SO₄²⁻ valency 2 → criss-cross se K ki subscript 2 → K₂SO₄
In formulae se compound ke naam batao: (i) NaBr (ii) CuO (iii) ZnS (iv) Fe₂O₃
(i) NaBr → Sodium bromide
(ii) CuO → Copper oxide (yahan Cu ki valency 2 hai, isliye ise cupric oxide bhi bola jata hai)
(iii) ZnS → Zinc sulphide
(iv) Fe₂O₃ → Iron(III) oxide / Ferric oxide
Valency kya hoti hai? Ammonia (NH₃) me nitrogen ki aur sulphur dioxide (SO₂) me sulphur ki valency batao.
Valency = kisi element ke ek atom ki combining capacity, yaani woh hydrogen (ya kisi standard element) ke kitne atoms se combine ho sakta hai (uski outermost shell ke electrons se decide hoti hai).
NH₃ me N ki valency: N, 3 H atoms se combine hota hai → valency = 3
SO₂ me S ki valency: O ki valency 2 hai aur S, 2 O atoms se bonded hai (2×2=4 charge balance) → valency = 4
Atomicity kise kehte hain? In molecules ki atomicity batao: (i) Argon (ii) Oxygen (iii) Ozone (iv) Phosphorus (P₄) (v) Sulphur (S₈)
Atomicity = ek molecule me present atoms ki total sankhya.
(i) Argon (Ar) → 1 (monatomic)
(ii) Oxygen (O₂) → 2 (diatomic)
(iii) Ozone (O₃) → 3 (triatomic)
(iv) Phosphorus (P₄) → 4 (tetra-atomic)
(v) Sulphur (S₈) → 8 (poly-atomic)
H₂, O₂, Cl₂ aur NH₃ ka molecular mass calculate karo. (Atomic mass: H=1, O=16, Cl=35.5, N=14)
H₂ = 2 × 1 = 2 u
O₂ = 2 × 16 = 32 u
Cl₂ = 2 × 35.5 = 71 u
NH₃ = 14 + (3 × 1) = 17 u
CaCl₂ aur Na₂SO₄ ki formula unit mass calculate karo. (Atomic mass: Ca=40, Cl=35.5, Na=23, S=32, O=16)
CaCl₂ = 40 + (2 × 35.5) = 40 + 71 = 111 u
Na₂SO₄ = (2 × 23) + 32 + (4 × 16) = 46 + 32 + 64 = 142 u
Note: ye dono ionic compounds hain, isliye "molecular mass" nahi "formula unit mass" bolte hain — inke discrete molecules nahi, balki crystal lattice me ions hote hain.
Sodium sulphate (Na₂SO₄) me har element ka mass percent calculate karo.
Step 1 — molar mass: Na₂SO₄ = 142 g/mol (upar calculate kiya)
Mass % of Na = (46/142) × 100 = 32.39%
Mass % of S = (32/142) × 100 = 22.54%
Mass % of O = (64/142) × 100 = 45.07%
Check: 32.39 + 22.54 + 45.07 ≈ 100% ✓
Important Equations — Ek Nazar Me
| Concept | Formula / Rule |
|---|---|
| Law of Conservation of Mass | Mass of reactants = Mass of products (koi mass create/destroy nahi hoti) |
| Law of Constant Proportions | Ek pure compound me elements hamesha fixed mass-ratio me present hote hain, chahe source kuch bhi ho |
| Atomic Mass Unit (u) | 1 u = (1/12) × mass of one atom of Carbon-12 |
| Molecular Mass | Sum of atomic masses of all atoms in one molecule (covalent compounds ke liye) |
| Formula Unit Mass | Sum of atomic masses of all atoms/ions in one formula unit (ionic compounds ke liye) |
| Chemical Formula Rule (criss-cross) | Ek element/ion ki valency doosre ki subscript ban jaati hai, phir subscripts ko lowest whole-number ratio me simplify karo |
| Avogadro's Number (NA) | NA = 6.022 × 10²³ particles/mol (12 g Carbon-12 me atoms ki sankhya) |
| Mole–Mass Relation | Number of moles (n) = Given mass ÷ Molar mass |
| Mole–Particle Relation | Number of moles (n) = Number of particles ÷ NA |
| Molar Mass | 1 mole substance ka mass in grams = us substance ka atomic/molecular/formula-unit mass (numerically same, unit u → g) |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
- Molecular mass vs formula unit mass mix karna: ionic compounds (NaCl, CaCl₂) ke liye "molecular mass" bolna galat hai — unke discrete molecules nahi hote, sirf ions hote hain, isliye "formula unit mass" hi sahi term hai.
- Criss-cross ke baad simplify karna bhool jaana: jaise Mg (valency 2) aur O (valency 2) ka formula Mg₂O₂ nahi, simplify karke MgO likhna hai — subscripts ko hamesha lowest whole-number ratio tak reduce karo.
- Polyatomic ion pe bracket lagana bhool jaana: jab polyatomic ion (NO₃⁻, SO₄²⁻, OH⁻) ki count 1 se zyada honi ho, to bracket zaroori hai — jaise Ca(NO₃)₂, na ki CaNO₃₂ ya CaN₂O₆ (jo galat interpretation degi).
- Atomic mass unit ki definition galat likhna: "1/12th mass of carbon atom" likh dena kaafi nahi — carbon ka specifically C-12 isotope reference hai, ye specify karna zaroori hai exam me full marks ke liye.
- Avogadro number sirf atoms ke liye use samajhna: NA = 6.022×10²³ kisi bhi particle — atom, molecule, ion — pe apply hota hai, sirf atoms tak limited nahi hai.
- Mole-to-mass conversion me molar mass ki unit confuse karna: moles nikalne ke liye hamesha grams ko molar mass (g/mol) se divide karo — agar mass ko galti se atomic mass (u) treat kar diya to answer 1000x ya usse bhi zyada galat aa sakta hai.
Board-Style Important Questions
- 1 mark: Law of conservation of mass ko define karo.
- 1 mark: Ek mole kisi bhi substance me kitne particles hote hain?
- 2 marks: Molecular mass aur formula unit mass me farak batao, ek-ek example ke saath.
- 2 marks: CaCO₃ (calcium carbonate) ka formula unit mass calculate karo. (Ca=40, C=12, O=16)
- 3 marks: Dalton ki atomic theory ke koi teen postulates likho, aur batao inme se kaunsa law of conservation of mass explain karta hai.
- 5 marks: (a) Chemical formula likhne ka criss-cross method valency ke saath samjhao, do examples ke saath. (b) 0.24 g magnesium, 0.16 g oxygen se combine hokar 0.40 g magnesium oxide banata hai. Dikhao ye law of conservation of mass follow karta hai.
Aksar Poochhe Jaane Wale Sawaal
Purane syllabus me is chapter ko "Atoms and Molecules" (class 9 science chapter 3 ncert solutions) bola jata tha — naye 2026-27 rationalised syllabus me isse chapter 9 "Atomic Foundations of Matter" bola ja raha hai?
Haan. Naye NCERT textbook "Exploration" (Grade 9 Science) me content largely same hai — laws of chemical combination, atoms, molecules, ions, chemical formula, mole concept — lekin chapter ka naam aur number badal gaya hai. Search karte waqt agar tumhe purana naam "Atoms and Molecules" mile to samajh lo woh isi chapter ki baat kar raha hai, kyunki transition period me student search behaviour abhi bhi purane naam se zyada hota hai.
Class 9 science ka new syllabus 2026-27 chapter list kya hai?
Naye "Exploration" textbook me total 13 chapters hain (Chapter 1 introductory bhi count hota hai). Is chapter (Ch.9 Atomic Foundations of Matter) se pehle Ch.8 "Journey Inside the Atom" (purana Structure of the Atom) aata hai — yaani sub-atomic particles (electron/proton/neutron) pehle padha diya jata hai, phir atomic mass/mole concept.
Atoms and molecules important questions class 9 me sabse zyada kya puchha jata hai?
Teen types dominate karte hain: (1) laws of chemical combination ke numericals (conservation of mass, constant proportions), (2) chemical formula writing with valency/criss-cross method, (3) mole concept numericals — molecular mass, formula unit mass, moles-to-grams conversion, aur Avogadro number wale questions. Formula table aur mistakes section neeche in sab ko cover karte hain.
Molecular mass aur formula unit mass me kya farak hai?
Molecular mass sirf un compounds ke liye use hota hai jinke discrete (covalent) molecules exist karte hain — jaise H₂O, CO₂. Formula unit mass ionic compounds (jaise NaCl, CaCl₂) ke liye use hota hai, kyunki wo molecules nahi banate — unke ions crystal lattice me arranged hote hain, isliye unke "smallest repeating unit" ka mass formula unit mass kehlata hai.
Class 9 science ke saare chapters ke notes PDF kahan milenge?
NCERT ki official website (ncert.nic.in) par naye textbook "Exploration" ke individual chapter PDFs free download available hain. Agar tum "class 9 science all chapters notes pdf" search kar rahe ho, to official NCERT PDF ko hi primary source maano — third-party notes me chapter-naming/numbering purane syllabus wala reh sakta hai jo confusion create karega.
Is chapter ke baad next chapter kya hai aur uska link kya hai?
Ye chapter (Ch.9, Atomic Foundations of Matter) mole concept aur chemical formula ki foundation deta hai. Isse pehle Ch.8 "Journey Inside the Atom" (structure of atom class 9 notes wala topic — electron, proton, neutron, atomic number) padhaya ja chuka hota hai. Dono chapters saath me Class 10 Chemistry (mole concept, stoichiometry, periodic table) ki neev bante hain.
Class 9 Science — Saare Chapters
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