โš›๏ธ Chapter 3: Atoms and Molecules ๐Ÿงฌ

๐ŸŒŸ The Building Blocks of Matter

Journey into the microscopic world where atoms dance and molecules form!

โš–๏ธLaws of Chemical Combination

๐Ÿ”’ Law of Conservation of Mass

Antoine Lavoisier (1789):
Mass can neither be created nor destroyed in a chemical reaction.
Example:
2H2 + O2 โ†’ 2H2O
Mass before = Mass after
4 + 32 = 36 grams

๐ŸŽฏ Law of Constant Proportion

Joseph Proust (1799):
In a chemical compound, elements are always present in definite proportions by mass.
Example:
Water (H2O) always contains:
H : O = 2 : 16 = 1 : 8 (by mass)
Whether from river, rain, or laboratory!

๐Ÿ”ฌDalton's Atomic Theory

John Dalton (1808) - The Atomic Pioneer

Revolutionized our understanding of matter with his atomic theory!

๐Ÿงฑ Postulate 1

All matter is made of tiny indivisible particles called atoms.

Modern Update: Atoms can be further divided into subatomic particles!

๐ŸŽญ Postulate 2

Atoms of the same element are identical in mass and properties.

Modern Update: Isotopes exist with different masses!

๐Ÿ”— Postulate 3

Atoms of different elements have different masses and properties.

Hydrogen โ‰  Oxygen โ‰  Carbon

๐Ÿงฎ Postulate 4

Atoms combine in simple whole-number ratios to form compounds.

H2O: 2 H atoms + 1 O atom

โš›๏ธ Postulate 5

Atoms can neither be created nor destroyed in chemical reactions.

Same number of atoms before and after reaction

๐Ÿท๏ธAtoms and Their Symbols

Atom: The smallest unit of an element that retains its chemical properties and cannot be broken down by chemical means.

๐Ÿ”ค Symbol Representation

Each element has a unique symbol, usually derived from its English or Latin name:

H

Hydrogen

Greek: Hydro + genes

O

Oxygen

Greek: Oxys + genes

Fe

Iron

Latin: Ferrum

Au

Gold

Latin: Aurum

Ag

Silver

Latin: Argentum

Na

Sodium

Latin: Natrium

K

Potassium

Latin: Kalium

Pb

Lead

Latin: Plumbum

โš—๏ธAtomic Mass and Mole Concept

Atomic Mass: The relative mass of an atom compared to 1/12th the mass of a carbon-12 atom.
Why Carbon-12?
โ€ข Abundant and stable isotope
โ€ข Exactly 12 atomic mass units
โ€ข Perfect reference standard

๐Ÿงช The Mole - Nature's Counting Unit

A mole is like a dozen, but much larger!

1 dozen = 12 items

1 mole = 6.022 ร— 1023 particles

๐ŸŽฏ Avogadro's Number

6.022 ร— 1023 = 602,200,000,000,000,000,000,000

Named after Amedeo Avogadro (Italian scientist)

๐Ÿงฎ Numerical Example

Question: What is the mass of 2 moles of CO2?

Solution:
Step 1: Find molar mass of CO2
โ€ข Carbon (C) = 12 u
โ€ข Oxygen (O) = 16 u
โ€ข CO2 = 12 + (16 ร— 2) = 12 + 32 = 44 g/mol

Step 2: Calculate mass
Mass = moles ร— molar mass
Mass = 2 ร— 44 = 88 grams

๐ŸงฌMolecules - When Atoms Team Up

Molecule: A group of two or more atoms chemically bonded together, representing the smallest unit of a compound.

๐ŸŽญ Types of Molecules

โš›๏ธ Monoatomic

Single atoms

Noble gases:

He, Ne, Ar, Kr, Xe

๐Ÿ”— Diatomic

Two atoms

H2, O2, N2

Cl2, F2, Br2, I2

๐ŸŒ Polyatomic

Many atoms

O3 (Ozone)

P4 (Phosphorus)

S8 (Sulfur)

Examples by Atomicity:
๐Ÿ”ธ Triatomic: H2O (water), CO2 (carbon dioxide), O3 (ozone)
๐Ÿ”ธ Tetratomic: NH3 (ammonia), P4 (white phosphorus)
๐Ÿ”ธ Octatomic: S8 (sulfur ring)

โšกIons - Charged Particles

Ion: An atom or molecule that has gained or lost electrons, resulting in a net electric charge.

โž• Cations (Positive Ions)

Formation: Loss of electrons

๐Ÿ”น Monovalent Cations (Charge = +1)

H+
Hydrogen
Li+
Lithium
Na+
Sodium
K+
Potassium
Rb+
Rubidium
Cs+
Cesium
Ag+
Silver
Au+
Gold (I)
Cu+
Copper (I)
NH4+
Ammonium

๐Ÿ”น Divalent Cations (Charge = +2)

Be2+
Beryllium
Mg2+
Magnesium
Ca2+
Calcium
Sr2+
Strontium
Ba2+
Barium
Ra2+
Radium
Zn2+
Zinc
Cd2+
Cadmium
Hg2+
Mercury (II)
Cu2+
Copper (II)
Fe2+
Iron (II)
Co2+
Cobalt (II)
Ni2+
Nickel (II)
Mn2+
Manganese (II)
Pb2+
Lead (II)
Sn2+
Tin (II)

๐Ÿ”น Trivalent Cations (Charge = +3)

Al3+
Aluminum
Ga3+
Gallium
In3+
Indium
Fe3+
Iron (III)
Cr3+
Chromium (III)
Co3+
Cobalt (III)
Au3+
Gold (III)
Bi3+
Bismuth
Memory Tip: CATions are PAWsitive! ๐Ÿฑ
Group 1 = +1, Group 2 = +2, Group 13 = +3

โž– Anions (Negative Ions)

Formation: Gain of electrons

๐Ÿ”น Monovalent Anions (Charge = โˆ’1)

Hโˆ’
Hydride
Fโˆ’
Fluoride
Clโˆ’
Chloride
Brโˆ’
Bromide
Iโˆ’
Iodide
OHโˆ’
Hydroxide
NO3โˆ’
Nitrate
NO2โˆ’
Nitrite
ClO3โˆ’
Chlorate
ClO4โˆ’
Perchlorate
MnO4โˆ’
Permanganate
CNโˆ’
Cyanide
SCNโˆ’
Thiocyanate
CH3COOโˆ’
Acetate
HCO3โˆ’
Bicarbonate
HSO4โˆ’
Bisulfate

๐Ÿ”น Divalent Anions (Charge = โˆ’2)

O2โˆ’
Oxide
S2โˆ’
Sulfide
Se2โˆ’
Selenide
Te2โˆ’
Telluride
SO42โˆ’
Sulfate
SO32โˆ’
Sulfite
CO32โˆ’
Carbonate
CrO42โˆ’
Chromate
Cr2O72โˆ’
Dichromate
HPO42โˆ’
Monohydrogen phosphate
C2O42โˆ’
Oxalate
SiO32โˆ’
Silicate

๐Ÿ”น Trivalent Anions (Charge = โˆ’3)

N3โˆ’
Nitride
P3โˆ’
Phosphide
As3โˆ’
Arsenide
PO43โˆ’
Phosphate
AsO43โˆ’
Arsenate
BO33โˆ’
Borate
Memory Tip: ANions are Negative! โšก
Group 17 = โˆ’1, Group 16 = โˆ’2, Group 15 = โˆ’3

โœ๏ธWriting Chemical Formulae

๐ŸŽฏ Steps to Write Chemical Formula

  1. Write symbols of elements/ions
  2. Write their valencies below the symbols
  3. Criss-cross the valencies
  4. Simplify if needed

๐Ÿ“‹ Common Valencies

Element/Ion Symbol Valency Element/Ion Symbol Valency
HydrogenH1 ChlorineCl1
SodiumNa1 OxygenO2
PotassiumK1 CalciumCa2
SilverAg1 MagnesiumMg2
AluminumAl3 Iron (II)Fe2
Iron (III)Fe3 CarbonC4

๐Ÿงฎ Formula Writing Examples

Example 1: Sodium Chloride
Na+1 + Clโˆ’1
Criss-cross: Na1Cl1
Formula: NaCl

Example 2: Calcium Chloride
Ca+2 + Clโˆ’1
Criss-cross: Ca1Cl2
Formula: CaCl2

Example 3: Aluminum Oxide
Al+3 + Oโˆ’2
Criss-cross: Al2O3
Formula: Al2O3

โš–๏ธMolecular Mass

Molecular Mass: The sum of atomic masses of all atoms present in one molecule of a compound.

๐Ÿงฎ Molecular Mass Calculations

Example 1: Water (H2O)
Molecular mass = (2 ร— 1) + (1 ร— 16)
= 2 + 16 = 18 u

Example 2: Sulfuric Acid (H2SO4)
Molecular mass = (2 ร— 1) + (1 ร— 32) + (4 ร— 16)
= 2 + 32 + 64 = 98 u

Example 3: Glucose (C6H12O6)
Molecular mass = (6 ร— 12) + (12 ร— 1) + (6 ร— 16)
= 72 + 12 + 96 = 180 u

๐ŸงชMole Relationships

๐ŸŒก๏ธ Standard Temperature and Pressure (STP)

Temperature: 0ยฐC (273 K)

Pressure: 1 atmosphere (760 mmHg)

๐ŸŽฏ Key Mole Relationships

๐Ÿงฎ Comprehensive Examples

Example 1: Number of Molecules
Q: How many molecules in 2 moles of NH3?
Solution: 2 ร— 6.022 ร— 1023
= 1.204 ร— 1024 molecules

Example 2: Mass from Moles
Q: What is the mass of 0.5 moles of CaCO3?
Solution: Molar mass of CaCO3 = 40 + 12 + (16 ร— 3) = 100 g/mol
Mass = 0.5 ร— 100 = 50 grams

Example 3: Volume at STP
Q: What volume does 2 moles of CO2 occupy at STP?
Solution: 2 ร— 22.4 L = 44.8 L

Example 4: Number of Atoms
Q: How many oxygen atoms in 1 mole of H2SO4?
Solution: 1 molecule has 4 oxygen atoms
1 mole = 6.022 ร— 1023 molecules
Oxygen atoms = 4 ร— 6.022 ร— 1023
Example 11: Potassium Permanganate
Step 1: Write symbols โ†’ K+ + MnO4โˆ’
Step 2: Criss-cross โ†’ K1(MnO4)1
Step 3: Final formula โ†’ KMnO4
โœ… Check: (+1) + (โˆ’1) = 0 โœ“

Example 12: Calcium Hydrogen Phosphate
Step 1: Write symbols โ†’ Ca2+ + HPO42โˆ’
Step 2: Criss-cross โ†’ Ca2(HPO4)2
Step 3: Simplify โ†’ CaHPO4
โœ… Check: (+2) + (โˆ’2) = 0 โœ“

๐ŸŽ“ Formula Writing Practice Exercise

Try writing formulas for these compounds using the criss-cross method!

1. Sodium Hydroxide
Na+ + OHโˆ’ = ?
2. Barium Chloride
Ba2+ + Clโˆ’ = ?
3. Iron(III) Sulfate
Fe3+ + SO42โˆ’ = ?
4. Ammonium Carbonate
NH4+ + CO32โˆ’ = ?
Answers: 1. NaOH 2. BaCl2 3. Fe2(SO4)3 4. (NH4)2CO3

๐Ÿ† Advanced Formula Writing Challenges

๐Ÿ”น Hydrated Compounds

Copper(II) Sulfate Pentahydrate: CuSO4 ยท 5H2O
Washing Soda: Na2CO3 ยท 10H2O
Epsom Salt: MgSO4 ยท 7H2O
Gypsum: CaSO4 ยท 2H2O

๐Ÿ”น Acid Salts (Multiple H+ ions)

Sodium Hydrogen Carbonate: NaHCO3 (Baking soda)
Potassium Hydrogen Sulfate: KHSO4
Calcium Hydrogen Phosphate: CaHPO4
Sodium Dihydrogen Phosphate: NaH2PO4

๐Ÿ”น Double Salts

Alum: K2SO4 ยท Al2(SO4)3 ยท 24H2O
Mohr's Salt: (NH4)2SO4 ยท FeSO4 ยท 6H2O
Carnallite: KCl ยท MgCl2 ยท 6H2O

๐Ÿ”น Coordination Compounds

Potassium Ferricyanide: K3[Fe(CN)6]
Sodium Ferrocyanide: Na4[Fe(CN)6] ยท 10H2O
Tetraammine Copper(II) Sulfate: [Cu(NH3)4]SO4

๐Ÿงช Common Compound Names and Formulas

Important Compounds for Class IX
Common Name Chemical Name Formula Use
Table Salt Sodium Chloride NaCl Food preservation
Baking Soda Sodium Hydrogen Carbonate NaHCO3 Cooking, antacid
Washing Soda Sodium Carbonate Decahydrate Na2CO3 ยท 10H2O Laundry, glass making
Caustic Soda Sodium Hydroxide NaOH Soap making
Lime Water Calcium Hydroxide Ca(OH)2 CO2 detection
Plaster of Paris Calcium Sulfate Hemihydrate CaSO4 ยท ยฝH2O Medical casts
Epsom Salt Magnesium Sulfate Heptahydrate MgSO4 ยท 7H2O Medicine, bath salts
Blue Vitriol Copper(II) Sulfate Pentahydrate CuSO4 ยท 5H2O Fungicide
Green Vitriol Iron(II) Sulfate Heptahydrate FeSO4 ยท 7H2O Iron supplement
Chile Saltpetre Sodium Nitrate NaNO3 Fertilizer
Indian Saltpetre Potassium Nitrate KNO3 Gunpowder, fertilizer
Gypsum Calcium Sulfate Dihydrate CaSO4 ยท 2H2O Cement, plaster
Marble Calcium Carbonate CaCO3 Construction
Limestone Calcium Carbonate CaCO3 Cement making
Chalk Calcium Carbonate CaCO3 Writing, antacid

๐Ÿ“‹ Comprehensive Valency Table

Common Elements and Their Valencies
Element Symbol Valency Element Symbol Valency
HydrogenH1 ChlorineCl1
LithiumLi1 FluorineF1
SodiumNa1 BromineBr1
PotassiumK1 IodineI1
SilverAg1 OxygenO2
BerylliumBe2 SulfurS2
MagnesiumMg2 NitrogenN3
CalciumCa2 PhosphorusP3, 5
ZincZn2 CarbonC4
IronFe2, 3 SiliconSi4
CopperCu1, 2 LeadPb2, 4
AluminumAl3 TinSn2, 4

๐Ÿ“‹ Common Polyatomic Ions and Their Valencies

Polyatomic Ions
Ion Name Formula Charge Ion Name Formula Charge
AmmoniumNH4++1 HydroxideOHโˆ’โˆ’1
HydroniumH3O++1 NitrateNO3โˆ’โˆ’1
Mercury(I)Hg22++2 NitriteNO2โˆ’โˆ’1
CarbonateCO32โˆ’โˆ’2 BicarbonateHCO3โˆ’โˆ’1
SulfateSO42โˆ’โˆ’2 BisulfateHSO4โˆ’โˆ’1
SulfiteSO32โˆ’โˆ’2 AcetateCH3COOโˆ’โˆ’1
PhosphatePO43โˆ’โˆ’3 PermanganateMnO4โˆ’โˆ’1
PhosphitePO33โˆ’โˆ’3 ChlorateClO3โˆ’โˆ’1
ChromateCrO42โˆ’โˆ’2 PerchlorateClO4โˆ’โˆ’1
DichromateCr2O72โˆ’โˆ’2 CyanideCNโˆ’โˆ’1

๐Ÿงฎ Detailed Formula Writing Examples

๐Ÿ”น Simple Binary Compounds

Example 1: Sodium Chloride
Step 1: Write symbols โ†’ Na + Cl
Step 2: Write valencies โ†’ Na+1 + Clโˆ’1
Step 3: Criss-cross โ†’ Na1Cl1
Step 4: Simplify โ†’ NaCl
โœ… Check: (+1) + (โˆ’1) = 0 โœ“

Example 2: Magnesium Oxide
Step 1: Write symbols โ†’ Mg + O
Step 2: Write valencies โ†’ Mg+2 + Oโˆ’2
Step 3: Criss-cross โ†’ Mg2O2
Step 4: Simplify โ†’ MgO (divide by 2)
โœ… Check: (+2) + (โˆ’2) = 0 โœ“

Example 3: Aluminum Chloride
Step 1: Write symbols โ†’ Al + Cl
Step 2: Write valencies โ†’ Al+3 + Clโˆ’1
Step 3: Criss-cross โ†’ Al1Cl3
Step 4: Final formula โ†’ AlCl3
โœ… Check: (+3) + 3(โˆ’1) = 0 โœ“

๐Ÿ”น Compounds with Polyatomic Ions

Example 4: Calcium Sulfate
Step 1: Write symbols โ†’ Ca2+ + SO42โˆ’
Step 2: Criss-cross โ†’ Ca2(SO4)2
Step 3: Simplify โ†’ CaSO4
โœ… Check: (+2) + (โˆ’2) = 0 โœ“

Example 5: Aluminum Sulfate
Step 1: Write symbols โ†’ Al3+ + SO42โˆ’
Step 2: Criss-cross โ†’ Al2(SO4)3
Step 3: Final formula โ†’ Al2(SO4)3
โœ… Check: 2(+3) + 3(โˆ’2) = +6 โˆ’ 6 = 0 โœ“

Example 6: Ammonium Phosphate
Step 1: Write symbols โ†’ NH4+ + PO43โˆ’
Step 2: Criss-cross โ†’ (NH4)3PO4
Step 3: Final formula โ†’ (NH4)3PO4
โœ… Check: 3(+1) + (โˆ’3) = 0 โœ“

๐Ÿ”น Complex Examples

Example 7: Iron(III) Hydroxide
Step 1: Identify iron(III) โ†’ Fe3+
Step 2: Write symbols โ†’ Fe3+ + OHโˆ’
Step 3: Criss-cross โ†’ Fe(OH)3
Step 4: Final formula โ†’ Fe(OH)3
โœ… Check: (+3) + 3(โˆ’1) = 0 โœ“

Example 8: Copper(II) Nitrate
Step 1: Identify copper(II) โ†’ Cu2+
Step 2: Write symbols โ†’ Cu2+ + NO3โˆ’
Step 3: Criss-cross โ†’ Cu(NO3)2
Step 4: Final formula โ†’ Cu(NO3)2
โœ… Check: (+2) + 2(โˆ’1) = 0 โœ“

Example 9: Ammonium Dichromate
Step 1: Write symbols โ†’ NH4+ + Cr2O72โˆ’
Step 2: Criss-cross โ†’ (NH4)2Cr2O7
Step 3: Final formula โ†’ (NH4)2Cr2O7
โœ… Check: 2(+1) + (โˆ’2) = 0 โœ“

๐Ÿ”น Special Cases and Rules

Important Rules:
๐Ÿ”ธ Brackets: Use brackets when polyatomic ion has subscript > 1
๐Ÿ”ธ Roman Numerals: Indicate variable valency (Fe(II), Fe(III))
๐Ÿ”ธ Simplification: Always reduce to simplest ratio
๐Ÿ”ธ Charge Balance: Total positive = Total negative
๐Ÿ”ธ Order: Cation first, then anion
๐Ÿ”ธ Naming: Metal + Non-metal + "ide" (for simple compounds)
Example 10: Lead(IV) Oxide (Variable Valency)
Step 1: Identify lead(IV) โ†’ Pb4+
Step 2: Write symbols โ†’ Pb4+ + O2โˆ’
Step 3: Criss-cross โ†’ Pb2O4
Step 4: Simplify โ†’ PbO2
โœ… Check: (+4) + 2(โˆ’2) = 0 โœ“

Example 11: Potassium Permanganate
Step 1: Write symbols โ†’ K+ + MnO4 = 2.408 ร— 1024 atoms

๐Ÿง Practice Questions (MCQs)

1. Which law states that mass is conserved in a chemical reaction?
A) Dalton's law
B) Boyle's law
C) Law of conservation of mass
D) Avogadro's law
2. Dalton's atomic theory does NOT include:
A) Atoms are indivisible
B) Atoms are identical for same element
C) Atoms can be created in reactions
D) Atoms combine in whole number ratios
3. The symbol for iron is:
A) I
B) Fe
C) Ir
D) In
4. Atomic mass of oxygen is:
A) 12 u
B) 14 u
C) 16 u
D) 18 u
5. Molecular mass of CO2 is:
A) 28 u
B) 32 u
C) 44 u
D) 52 u
6. One mole of a substance contains:
A) 6.022 ร— 1023 particles
B) 1 gram of substance
C) 22.4 L of gas
D) 1000 particles
7. Formula of aluminum chloride is:
A) AlCl
B) AlCl2
C) AlCl3
D) Al2Cl3
8. Which is a triatomic molecule?
A) O2
B) H2O
C) He
D) P4
9. Mass of 2 moles of CO2 is:
A) 22 g
B) 44 g
C) 88 g
D) 176 g
10. Which one is an anion?
A) Na+
B) Ca2+
C) Clโˆ’
D) Al3+
11. Avogadro's number refers to:
A) Only atoms
B) Only ions
C) Only molecules
D) Any particles
12. Which of the following is NOT a molecule?
A) H2
B) N2
C) Fe
D) O3
13. One mole of gas at STP occupies:
A) 22.4 mL
B) 22.4 L
C) 2.24 L
D) 1 L
14. H2SO4 is called:
A) Hydrochloric acid
B) Nitric acid
C) Sulfuric acid
D) Acetic acid
15. Fe2O3 is:
A) Iron chloride
B) Iron oxide
C) Ferric oxide
D) Both B and C
16. The valency of calcium is:
A) 1
B) 2
C) 3
D) 4
17. Which is a polyatomic molecule?
A) S8
B) O2
C) H2
D) CO
18. Chemical formula of sodium sulfate is:
A) NaSO4
B) Na2SO4
C) NaSO3
D) Na2SO3
19. Atomicity of O3 is:
A) 1
B) 2
C) 3
D) 4
20. Which has the greatest number of atoms?
A) 2 mol H2
B) 1 mol O2
C) 3 mol He
D) 1 mol CO2
21. Mass of 1 mole of water is:
A) 18 g
B) 16 g
C) 20 g
D) 12 g
22. 1 mole of H2O contains how many atoms?
A) 6.022 ร— 1023
B) 1.204 ร— 1024
C) 1.806 ร— 1024
D) 3.011 ร— 1023
23. Which has the highest molecular mass?
A) H2 (2 u)
B) CO2 (44 u)
C) SO2 (64 u)
D) CH4 (16 u)
24. Atomic mass unit (u) is based on:
A) H-1
B) C-12
C) O-16
D) N-14
25. MgCl2 contains:
A) 1 Mg, 1 Cl
B) 1 Mg, 2 Cl
C) 2 Mg, 1 Cl
D) 2 Mg, 2 Cl
26. 2 moles of H2O contain:
A) 3.6 ร— 1024 atoms
B) 6.022 ร— 1023 molecules
C) 1.204 ร— 1024 molecules
D) 1.806 ร— 1024 atoms
27. Which is a cation?
A) Clโˆ’
B) OHโˆ’
C) NH4+
D) SO42โˆ’
28. Number of moles in 44 g CO2:
A) 0.5
B) 1
C) 2
D) 4
29. Al2(SO4)3 contains:
A) 2 Al, 3 S, 12 O
B) 1 Al, 1 S, 4 O
C) 2 Al, 1 S, 4 O
D) 1 Al, 3 S, 12 O
30. HCl contains:
A) Hydrogen and oxygen
B) Hydrogen and chlorine
C) Hydrogen and nitrogen
D) Hydrogen and fluorine
31. The molecular mass of CaCO3 is:
A) 90 u
B) 100 u
C) 110 u
D) 120 u
32. Which gas has the maximum number of molecules in 1 mole?
A) H2
B) O2
C) CO2
D) All have same number
33. The formula of magnesium oxide is:
A) MgO2
B) MgO
C) Mg2O
D) Mg2O3
34. Volume occupied by 0.5 moles of gas at STP:
A) 11.2 L
B) 22.4 L
C) 44.8 L
D) 5.6 L
35. The number of oxygen atoms in 1 mole of Al2(SO4)3:
A) 12 ร— 6.022 ร— 1023
B) 4 ร— 6.022 ร— 1023
C) 6.022 ร— 1023
D) 3 ร— 6.022 ร— 1023

โœ…Answer Key

1. C 2. C 3. B 4. C 5. C 6. A 7. C
8. B 9. C 10. C 11. D 12. C 13. B 14. C
15. D 16. B 17. A 18. B 19. C 20. A 21. A
22. C 23. C 24. B 25. B 26. C 27. C 28. B
29. A 30. B 31. B 32. D 33. B 34. A 35. A

๐ŸŽฏKey Takeaways

๐Ÿ”ฌAmazing Atomic Facts!

ยฉ 2025 Enhanced Educational Content | Class IX NCERT Science | Chapter 3: Atoms and Molecules
Exploring the Invisible World of Matter! โš›๏ธ๐Ÿงชโœจ

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