GS Paper III · Science & Technology · Basic Chemistry · NCERT Class 9
⚛ Atoms & Molecules — Complete UPSC Notes
Laws of Chemical Combination · Dalton's Atomic Theory · Molecules, Ions & Valency · Atomic Models (Thomson / Rutherford / Bohr) · Isotopes & Isobars · Mole Concept · PYQs & MCQs
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Laws of Chemical Combination
Law of conservation of mass · Law of constant proportions · Lavoisier · Proust
⚖ Law of Conservation of Mass
Statement: Mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants equals total mass of products.
Example: If you burn 12 g of carbon in 32 g of oxygen → you always get exactly 44 g of carbon dioxide. Nothing is lost — atoms are merely rearranged.
Discoverer: Antoine Lavoisier (French chemist, "father of modern chemistry")
UPSC relevance: Basis of Dalton's atomic theory. Also the foundation of stoichiometry (balancing chemical equations).
Example: If you burn 12 g of carbon in 32 g of oxygen → you always get exactly 44 g of carbon dioxide. Nothing is lost — atoms are merely rearranged.
Discoverer: Antoine Lavoisier (French chemist, "father of modern chemistry")
UPSC relevance: Basis of Dalton's atomic theory. Also the foundation of stoichiometry (balancing chemical equations).
📐 Law of Definite (Constant) Proportions
Statement (Proust): "In a chemical substance, the elements are always present in definite proportions by mass — regardless of source or method of preparation."
Example: Water always has hydrogen : oxygen = 1:8 by mass. Whether water comes from rain, a river, or a lab — 9 g of water always gives 1 g H + 8 g O. You can't make water with a different H:O ratio.
Also called: Law of Definite Proportions
Discoverer: Joseph Proust (French chemist, 1799)
Example: Water always has hydrogen : oxygen = 1:8 by mass. Whether water comes from rain, a river, or a lab — 9 g of water always gives 1 g H + 8 g O. You can't make water with a different H:O ratio.
Also called: Law of Definite Proportions
Discoverer: Joseph Proust (French chemist, 1799)
🧠 Simple Analogy — LEGO blocks
Imagine atoms are LEGO blocks of specific sizes and colours. To build a "water molecule" LEGO model, you ALWAYS need exactly 2 blue blocks (H) and 1 red block (O) — no matter where you get the bricks or who builds it. This is the law of constant proportions. And when you take apart the model, all the original blocks are still there — no blocks created or destroyed. That's conservation of mass.
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Dalton's Atomic Theory (1808) — & Its Limitations
Postulates · Atom as indivisible · Symbols · IUPAC · Failures
📖 Dalton's Postulates (1808)
John Dalton, British chemist, published his Atomic Theory in 1808 — the first systematic atomic theory. He built on the two laws above to propose:
- All matter is made of very tiny particles called atoms
- Atoms are indivisible — cannot be created or destroyed in a chemical reaction
- All atoms of a given element are identical in mass and chemical properties
- Atoms of different elements have different masses and properties
- Atoms combine in small whole number ratios to form compounds
- The relative number and kinds of atoms are constant in a given compound
Dalton's Atomic Model — a solid, indivisible sphere with no internal structure. Successfully explained the two laws of chemical combination. Failed when sub-atomic particles (electron by Thomson, proton by Goldstein, neutron by Chadwick) were discovered. Element symbols: proposed by Dalton (pictures), improved by Berzelius (letters), standardised by IUPAC. Some symbols come from Latin names: Fe (Ferrum=Iron), Au (Aurum=Gold), Na (Natrium=Sodium), Cu (Cuprum=Copper), Ag (Argentum=Silver).
✅ Successes of Dalton's Theory
• Explains Law of Conservation of Mass (atoms not created/destroyed)
• Explains Law of Definite Proportions (fixed atom ratios → fixed mass ratios)
• Introduced atomic mass concept
• Introduced element symbols (Berzelius improved; IUPAC standardised)
• Foundation for all modern chemistry
• Explains Law of Definite Proportions (fixed atom ratios → fixed mass ratios)
• Introduced atomic mass concept
• Introduced element symbols (Berzelius improved; IUPAC standardised)
• Foundation for all modern chemistry
❌ Limitations / Failures
• Atoms ARE divisible — sub-atomic particles exist (e⁻, p⁺, n°)
• Atoms of same element can have different masses (isotopes)
• Atoms of different elements can have same mass (isobars)
• Does not explain allotropes (diamond vs graphite — both carbon)
• Does not explain compounds formed with non-integer ratios (Berthollet found this)
• Atoms of same element can have different masses (isotopes)
• Atoms of different elements can have same mass (isobars)
• Does not explain allotropes (diamond vs graphite — both carbon)
• Does not explain compounds formed with non-integer ratios (Berthollet found this)
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Structure of the Atom — Sub-atomic Particles
Electron · Proton · Neutron · Nucleus · Valency · Atomic number · Mass number
| Particle | Discoverer | Charge | Mass | Location | Symbol |
|---|---|---|---|---|---|
| Electron | J.J. Thomson (1897) Nobel Physics 1906 | −1 (negative) | ~1/2000 of proton; negligible | Orbits / shells around nucleus | e⁻ |
| Proton | Goldstein (1886, canal rays) | +1 (positive) | ~1 u (atomic mass unit) | Inside nucleus | p⁺ |
| Neutron | James Chadwick (1932) Nobel Physics 1935 | 0 (neutral) | ~1 u (≈ proton mass) | Inside nucleus | n° |
Carbon-12 Atom Structure. Nucleus (red) contains 6 protons (p⁺, red) + 6 neutrons (n°, grey). K-shell (inner, orange electrons): 2 electrons. L-shell (outer, green electrons): 4 electrons. Shell capacity formula: 2n² (K=2, L=8, M=18, N=32). Outermost shell maximum: 8 electrons. Atomic number Z = number of protons = 6. Mass number A = p + n = 12. Neutrons = A − Z = 12 − 6 = 6. Valency of Carbon = 4 (needs 4 more electrons to complete octet).
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Atomic Number (Z)
= Total protons in nucleus. Defines the element — every element has a unique Z. All atoms of same element have same Z. Neutral atom: Z = number of electrons.
H: Z=1, He: Z=2, Li: Z=3, C: Z=6, N: Z=7, O: Z=8, Na: Z=11, Mg: Z=12, Al: Z=13, Cl: Z=17, Ca: Z=20.
H: Z=1, He: Z=2, Li: Z=3, C: Z=6, N: Z=7, O: Z=8, Na: Z=11, Mg: Z=12, Al: Z=13, Cl: Z=17, Ca: Z=20.
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Mass Number (A)
= Protons + Neutrons (nucleons). Electron mass is negligible → mass is practically all in nucleus.
Notation: ᴬ꜀Z X where X = element symbol, A = mass number, Z = atomic number.
Carbon-12: ¹²C₆ → 6p + 6n. Carbon-14: ¹⁴C₆ → 6p + 8n. Both are carbon!
Notation: ᴬ꜀Z X where X = element symbol, A = mass number, Z = atomic number.
Carbon-12: ¹²C₆ → 6p + 6n. Carbon-14: ¹⁴C₆ → 6p + 8n. Both are carbon!
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Valency
Combining capacity of an atom. Number of electrons gained, lost, or shared to achieve a full outer shell (octet = 8 electrons).
H: 1, He: 0, Li: 1, C: 4, N: 3, O: 2, F: 1, Na: 1, Mg: 2, Al: 3, Cl: 1, Ca: 2
Rule: If outer electrons >4, valency = 8 − outer electrons (gains). If <4, valency = outer electrons (loses).
H: 1, He: 0, Li: 1, C: 4, N: 3, O: 2, F: 1, Na: 1, Mg: 2, Al: 3, Cl: 1, Ca: 2
Rule: If outer electrons >4, valency = 8 − outer electrons (gains). If <4, valency = outer electrons (loses).
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Atomic Models — Thomson → Rutherford → Bohr High Yield
Plum pudding · Gold foil experiment · Nuclear model · Electron shells · Drawbacks
Three Atomic Models — Evolution. Thomson (1904): Positive charge spread throughout sphere, electrons embedded like plum in pudding (watermelon analogy). ❌ Disproved by Rutherford's gold foil experiment. Rutherford (1911): Tiny dense positive nucleus at centre, electrons orbiting it. ❌ Electrons should spiral into nucleus (Maxwell's laws — accelerating charges radiate energy). Bohr (1913): Electrons in specific fixed orbits (shells K, L, M, N) with discrete energies — they don't radiate energy while in these fixed orbits. ✅ Solved Rutherford's stability problem. All three scientists received Nobel Prizes.
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Thomson's Model
J.J. Thomson (1856–1940) — Nobel 1906
Discovered electron (1897) via cathode ray experiments.
Model: Positive charge uniformly spread like watermelon flesh. Electrons (negative) embedded like seeds in watermelon / plums in pudding. Atom overall neutral.
Success: Explained electrical neutrality.
Failure: Rutherford's experiment showed positive charge concentrated, not spread.
Discovered electron (1897) via cathode ray experiments.
Model: Positive charge uniformly spread like watermelon flesh. Electrons (negative) embedded like seeds in watermelon / plums in pudding. Atom overall neutral.
Success: Explained electrical neutrality.
Failure: Rutherford's experiment showed positive charge concentrated, not spread.
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Rutherford's Model & Gold Foil Experiment
Ernest Rutherford — Nobel Chemistry 1908
Experiment: α-particles (doubly charged He ions) fired at thin gold foil (~1000 atoms thick).
Observations:
• Most passed straight through → atom mostly empty space
• Some deflected at small angles → small positive charge region
• 1 in 12,000 bounced back → tiny dense positive nucleus
Failure: Accelerating electrons should radiate energy → spiral into nucleus → atom should collapse. Rutherford's atom is unstable!
Experiment: α-particles (doubly charged He ions) fired at thin gold foil (~1000 atoms thick).
Observations:
• Most passed straight through → atom mostly empty space
• Some deflected at small angles → small positive charge region
• 1 in 12,000 bounced back → tiny dense positive nucleus
Failure: Accelerating electrons should radiate energy → spiral into nucleus → atom should collapse. Rutherford's atom is unstable!
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Bohr's Model
Niels Bohr — Nobel Physics 1922
Postulates:
• Electrons travel only in specific fixed orbits (shells) — K, L, M, N (or n=1,2,3,4)
• While in these orbits, electrons do NOT radiate energy
• Energy is emitted/absorbed only when electrons jump between orbits
• Shell capacity: 2n² (K=2, L=8, M=18, N=32)
• Outermost shell max = 8 electrons
Solved: Stability problem of Rutherford's model.
Postulates:
• Electrons travel only in specific fixed orbits (shells) — K, L, M, N (or n=1,2,3,4)
• While in these orbits, electrons do NOT radiate energy
• Energy is emitted/absorbed only when electrons jump between orbits
• Shell capacity: 2n² (K=2, L=8, M=18, N=32)
• Outermost shell max = 8 electrons
Solved: Stability problem of Rutherford's model.
🎯 Rutherford's Gold Foil Experiment — UPSC Favourite
Setup: Thin gold foil (chosen because gold is malleable — can be made ~1000 atoms thick). α-particles (fast, heavy, +2 charge) fired at it from a radioactive source. Circular zinc sulphide screen detects where α-particles land (flashes of light).
Quote: Rutherford described the 1-in-12,000 rebound: "This result was almost as incredible as if you fire a 15-inch shell at a piece of tissue paper and it comes back and hits you."
Quote: Rutherford described the 1-in-12,000 rebound: "This result was almost as incredible as if you fire a 15-inch shell at a piece of tissue paper and it comes back and hits you."
Conclusions:
• Atom is mostly empty space (most α passed through)
• Small, dense, positively charged nucleus exists at centre
• Nucleus size ~10⁻⁵ times the atom's radius
• Electrons revolve around nucleus in orbits
Key discovery: The atomic nucleus — revolutionised understanding of matter. Rutherford rightly called "father of nuclear physics."
• Atom is mostly empty space (most α passed through)
• Small, dense, positively charged nucleus exists at centre
• Nucleus size ~10⁻⁵ times the atom's radius
• Electrons revolve around nucleus in orbits
Key discovery: The atomic nucleus — revolutionised understanding of matter. Rutherford rightly called "father of nuclear physics."
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Isotopes, Isobars & Applications
Same Z different A · Same A different Z · Hydrogen isotopes · Chlorine · Medical uses
Isotopes vs Isobars. ISOTOPES (left): Hydrogen has 3 isotopes — Protium (¹H, 1p, no neutron), Deuterium (²H, 1p+1n), Tritium (³H, 1p+2n). Same element (Z=1 for all) but different mass numbers. Same chemical properties, different physical properties. ISOBARS (right): Calcium (Z=20) and Argon (Z=18) — both have mass number 40 but are completely different elements with different chemical properties.
🧬 Applications of Isotopes UPSC Favourite
Nuclear energy: Uranium-235 (²³⁵U) → nuclear fission → fuel for nuclear reactors and atomic bombs
Cancer treatment: Cobalt-60 (⁶⁰Co) → gamma radiation → kills cancer cells (cobalt therapy)
Thyroid/Goitre treatment: Iodine-131 (¹³¹I) → treats thyroid disorders; Iodine-123 → thyroid imaging
Carbon dating: Carbon-14 (¹⁴C) → radioactive decay at known rate → dates organic materials up to 50,000 years old
Medical imaging: Technetium-99m (⁹⁹ᵐTc) → most widely used radioactive isotope in nuclear medicine; SPECT scans
Heavy water: Deuterium (²H) + Oxygen → D₂O (heavy water) → moderator in nuclear reactors
Archaeological dating: Uranium-238 → Lead-206 decay (half-life 4.47 billion years) → dates rocks
Cancer treatment: Cobalt-60 (⁶⁰Co) → gamma radiation → kills cancer cells (cobalt therapy)
Thyroid/Goitre treatment: Iodine-131 (¹³¹I) → treats thyroid disorders; Iodine-123 → thyroid imaging
Carbon dating: Carbon-14 (¹⁴C) → radioactive decay at known rate → dates organic materials up to 50,000 years old
Medical imaging: Technetium-99m (⁹⁹ᵐTc) → most widely used radioactive isotope in nuclear medicine; SPECT scans
Heavy water: Deuterium (²H) + Oxygen → D₂O (heavy water) → moderator in nuclear reactors
Archaeological dating: Uranium-238 → Lead-206 decay (half-life 4.47 billion years) → dates rocks
📊 Isotopes vs Isobars — Quick Compare
| Feature | Isotopes | Isobars |
|---|---|---|
| Atomic No. (Z) | Same | Different |
| Mass No. (A) | Different | Same |
| Element | Same element | Different elements |
| Protons | Same | Different |
| Neutrons | Different | Different |
| Chemical properties | Similar (same e⁻ config) | Different |
| Example | ¹²C and ¹⁴C (both carbon) | ⁴⁰Ca and ⁴⁰Ar |
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Mole Concept & Avogadro's Number
6.022×10²³ · Avogadro constant · Atomic mass · Molecular mass · Formula unit mass
📖 What is a Mole?
A mole is the SI unit for amount of substance. 1 mole of any substance contains 6.022 × 10²³ particles (atoms, molecules, ions) — this is the Avogadro Constant (Nₐ), named after Italian scientist Amedeo Avogadro. The mole is defined as the amount of substance that contains the same number of particles as there are atoms in exactly 12 g of Carbon-12. In grams, 1 mole of a substance = its atomic/molecular mass in grams (called molar mass).
🧠 Why Do We Need the Mole? — The Dozen Analogy
When you buy eggs, you don't say "I want 13 eggs" — you say "I want 1 dozen" (12 eggs). Similarly, atoms are so incredibly tiny that counting individual atoms is impossible in practice. Chemists need a convenient "counting unit" for atoms. The mole is that unit — like a "dozen" but for atoms. 1 mole = 6.022 × 10²³ particles. Just as 1 dozen eggs always = 12 eggs regardless of the egg's size, 1 mole of carbon = 12 g and 1 mole of oxygen = 16 g — the mass differs because atoms have different masses, but the COUNT (6.022 × 10²³) is always the same.
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Atomic Mass
Relative mass of an atom compared to 1/12 of Carbon-12 atom. Unit: u (unified atomic mass unit) or amu.
1 u = 1/12 × mass of one C-12 atom
H = 1 u, C = 12 u, N = 14 u, O = 16 u, Na = 23 u, Mg = 24 u, Al = 27 u, S = 32 u, Cl = 35.5 u, Ca = 40 u, Fe = 56 u
1 u = 1/12 × mass of one C-12 atom
H = 1 u, C = 12 u, N = 14 u, O = 16 u, Na = 23 u, Mg = 24 u, Al = 27 u, S = 32 u, Cl = 35.5 u, Ca = 40 u, Fe = 56 u
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Molecular Mass
Sum of atomic masses of all atoms in one molecule.
Examples:
H₂O = 2(1) + 16 = 18 u
CO₂ = 12 + 2(16) = 44 u
H₂SO₄ = 2(1)+32+4(16) = 98 u
NaCl = 23+35.5 = 58.5 u
NH₃ = 14+3(1) = 17 u
Ca(OH)₂ = 40+2(16+1) = 74 u
Examples:
H₂O = 2(1) + 16 = 18 u
CO₂ = 12 + 2(16) = 44 u
H₂SO₄ = 2(1)+32+4(16) = 98 u
NaCl = 23+35.5 = 58.5 u
NH₃ = 14+3(1) = 17 u
Ca(OH)₂ = 40+2(16+1) = 74 u
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Mole Calculations
Molar mass = Molecular mass in grams = mass of 1 mole
No. of moles = Given mass ÷ Molar mass
No. of particles = Moles × 6.022 × 10²³
Example: 36 g of water = 36/18 = 2 moles = 2 × 6.022×10²³ = 1.204×10²⁴ molecules
Avogadro No. = 6.022 × 10²³ per mole
No. of moles = Given mass ÷ Molar mass
No. of particles = Moles × 6.022 × 10²³
Example: 36 g of water = 36/18 = 2 moles = 2 × 6.022×10²³ = 1.204×10²⁴ molecules
Avogadro No. = 6.022 × 10²³ per mole
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Molecules, Ions & Chemical Formulae
Atomicity · Cation · Anion · Polyatomic ion · Valency cross method · Writing formulae
⚛ Atomicity of Molecules
Atomicity = number of atoms in one molecule of an element.
Monoatomic (1): Noble gases — He, Ne, Ar, Kr, Xe, Rn (exist as single atoms)
Diatomic (2): H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂ (the "diatomic 7")
Triatomic (3): O₃ (ozone), H₂O, CO₂, SO₂
Tetraatomic (4): P₄ (white phosphorus), NH₃
Polyatomic: S₈ (sulphur — 8 atoms!), C₆₀ (Buckminsterfullerene)
Monoatomic (1): Noble gases — He, Ne, Ar, Kr, Xe, Rn (exist as single atoms)
Diatomic (2): H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂ (the "diatomic 7")
Triatomic (3): O₃ (ozone), H₂O, CO₂, SO₂
Tetraatomic (4): P₄ (white phosphorus), NH₃
Polyatomic: S₈ (sulphur — 8 atoms!), C₆₀ (Buckminsterfullerene)
⚡ Ions — Cations & Anions
Ion = charged particle (atom or group of atoms that has gained/lost electrons).
Cation (+): Loses electrons → positive charge. Na⁺, Mg²⁺, Al³⁺, Ca²⁺, H⁺, NH₄⁺
Anion (−): Gains electrons → negative charge. Cl⁻, O²⁻, N³⁻, OH⁻, SO₄²⁻, CO₃²⁻
Polyatomic ion: Group of atoms with net charge. OH⁻ (hydroxide), SO₄²⁻ (sulphate), CO₃²⁻ (carbonate), NO₃⁻ (nitrate), PO₄³⁻ (phosphate), NH₄⁺ (ammonium)
Cation (+): Loses electrons → positive charge. Na⁺, Mg²⁺, Al³⁺, Ca²⁺, H⁺, NH₄⁺
Anion (−): Gains electrons → negative charge. Cl⁻, O²⁻, N³⁻, OH⁻, SO₄²⁻, CO₃²⁻
Polyatomic ion: Group of atoms with net charge. OH⁻ (hydroxide), SO₄²⁻ (sulphate), CO₃²⁻ (carbonate), NO₃⁻ (nitrate), PO₄³⁻ (phosphate), NH₄⁺ (ammonium)
📝 Writing Chemical Formulae — Cross Method
Rules:
1. Metal written first, non-metal second
2. Valencies must balance (cross-multiply)
3. Polyatomic ions get brackets if subscript > 1
Cross method examples:
MgO: Mg²⁺ and O²⁻ → valencies 2,2 → ratio 1:1 → MgO
AlCl₃: Al³⁺ and Cl⁻ → valencies 3,1 → ratio 1:3 → AlCl₃
Ca(OH)₂: Ca²⁺ and OH⁻ → valencies 2,1 → Ca takes 1 OH, OH gets 2 → Ca(OH)₂ (brackets because 2 OH groups)
Na₂SO₄: Na⁺ and SO₄²⁻ → valencies 1,2 → 2 Na needed → Na₂SO₄
Fe₂O₃: Fe³⁺ and O²⁻ → valencies 3,2 → cross = Fe₂O₃
1. Metal written first, non-metal second
2. Valencies must balance (cross-multiply)
3. Polyatomic ions get brackets if subscript > 1
Cross method examples:
MgO: Mg²⁺ and O²⁻ → valencies 2,2 → ratio 1:1 → MgO
AlCl₃: Al³⁺ and Cl⁻ → valencies 3,1 → ratio 1:3 → AlCl₃
Ca(OH)₂: Ca²⁺ and OH⁻ → valencies 2,1 → Ca takes 1 OH, OH gets 2 → Ca(OH)₂ (brackets because 2 OH groups)
Na₂SO₄: Na⁺ and SO₄²⁻ → valencies 1,2 → 2 Na needed → Na₂SO₄
Fe₂O₃: Fe³⁺ and O²⁻ → valencies 3,2 → cross = Fe₂O₃
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PYQs & Practice MCQs
Atomic models · Isotopes · Mole concept · Laws of chemistry · Nuclear applications
📜 UPSC Pattern — Atomic Structure / Rutherford's Experiment
Pattern Q
Q. With reference to Rutherford's alpha-particle scattering experiment, which of the following conclusions is/are correct?
- Most of the space inside an atom is empty, as most alpha particles passed straight through the gold foil.
- The positive charge and most of the mass of an atom are concentrated in a very small central region called the nucleus.
- Electrons revolve around the nucleus in fixed orbits with discrete energy levels, and do not radiate energy while in these orbits.
- a) 1 and 2 only ✓
- b) 2 and 3 only
- c) 1 and 3 only
- d) 1, 2 and 3
Statement 1 CORRECT — Rutherford's conclusion: Most alpha particles passed straight through the gold foil with no deflection → the atom is mostly empty space. Gold was chosen because it can be made into an extremely thin foil (~1000 atoms thick), allowing the experiment to test what happens when fast, heavy, positively charged particles encounter atoms at close range.
Statement 2 CORRECT — Rutherford's conclusion: A tiny fraction (~1 in 12,000) of alpha particles bounced back at large angles, including some nearly straight back. This could only happen if they encountered a very tiny but extremely dense, positively charged region — the nucleus. Rutherford calculated the nucleus radius is about 10⁻⁵ times the atom's radius (i.e., if an atom were the size of a football stadium, the nucleus would be the size of a marble).
Statement 3 WRONG — This is Bohr's postulate, NOT Rutherford's conclusion: Rutherford's model had electrons revolving around the nucleus, but Rutherford's model could NOT explain why electrons don't spiral in (they should lose energy by radiation). It was Niels Bohr (1913) who solved this by proposing that electrons exist in fixed orbits with discrete energy levels and do not radiate energy while in these specific orbits. Rutherford's experiment only proved the nuclear structure — the shell/energy level concept is entirely Bohr's contribution.
Statement 2 CORRECT — Rutherford's conclusion: A tiny fraction (~1 in 12,000) of alpha particles bounced back at large angles, including some nearly straight back. This could only happen if they encountered a very tiny but extremely dense, positively charged region — the nucleus. Rutherford calculated the nucleus radius is about 10⁻⁵ times the atom's radius (i.e., if an atom were the size of a football stadium, the nucleus would be the size of a marble).
Statement 3 WRONG — This is Bohr's postulate, NOT Rutherford's conclusion: Rutherford's model had electrons revolving around the nucleus, but Rutherford's model could NOT explain why electrons don't spiral in (they should lose energy by radiation). It was Niels Bohr (1913) who solved this by proposing that electrons exist in fixed orbits with discrete energy levels and do not radiate energy while in these specific orbits. Rutherford's experiment only proved the nuclear structure — the shell/energy level concept is entirely Bohr's contribution.
🧪 Practice MCQs — Atoms & Molecules (Click to attempt)
Q1. Carbon-12 (¹²C) and Carbon-14 (¹⁴C) are isotopes. Which of the following correctly describes the difference between them?
- (a) ¹²C has 6 protons and ¹⁴C has 8 protons — they are different elements despite similar names
- (b) Both have 6 protons (same element, same Z=6) but ¹²C has 6 neutrons while ¹⁴C has 8 neutrons — they have the same chemical properties but ¹⁴C is radioactive and used for carbon dating to determine the age of ancient organic materials
- (c) ¹²C and ¹⁴C have the same number of neutrons but different numbers of electrons — they have different chemical properties and belong to different periods of the periodic table
- (d) ¹⁴C has a higher atomic number than ¹²C — it was created artificially in nuclear reactors and does not occur naturally
Isotopes are atoms of the SAME element (same atomic number Z = same number of protons) but with DIFFERENT mass numbers (different number of neutrons). Both ¹²C and ¹⁴C have Z = 6 (6 protons each — this is what makes them both carbon). ¹²C: A=12, Z=6, so neutrons = A−Z = 12−6 = 6 neutrons. ¹⁴C: A=14, Z=6, so neutrons = 14−6 = 8 neutrons. Since both have 6 electrons (neutral atoms), they have identical electron configurations → identical chemical properties. But their physical properties differ: ¹⁴C is unstable (radioactive) and undergoes beta decay with a half-life of ~5,730 years. This is the basis of radiocarbon dating: living organisms continuously absorb ¹⁴C from the atmosphere. When an organism dies, no more ¹⁴C is absorbed, and the existing ¹⁴C starts decaying at the known rate. By measuring the remaining ratio of ¹⁴C to ¹²C, scientists can calculate how long ago the organism died — accurate up to about 50,000 years. ¹⁴C does occur naturally (created by cosmic ray bombardment of nitrogen in the upper atmosphere). ¹⁶O and ¹⁸O (oxygen isotopes) are also important for climate studies.
Q2. A student dissolves 9 g of water in a beaker. According to the Mole Concept, which of the following statements is correct?
- (a) The beaker contains exactly 1 mole of water molecules because the molar mass of water is 9 g/mol (hydrogen=1, oxygen=8, total=9)
- (b) The beaker contains 2 moles of water, giving 1.204 × 10²⁴ water molecules, because 9 g divided by the molar mass of 4.5 g/mol gives 2
- (c) The beaker contains 0.5 moles of water molecules (9 g ÷ 18 g/mol = 0.5 mol), which equals 0.5 × 6.022 × 10²³ = 3.011 × 10²³ water molecules; and since each H₂O has 2 hydrogen and 1 oxygen atom, it contains 6.022 × 10²³ hydrogen atoms and 3.011 × 10²³ oxygen atoms
- (d) The mole concept only applies to gases, not to liquids like water — so the mole calculation is not applicable here
Step-by-step solution: Molecular mass of water (H₂O) = 2(atomic mass of H) + 1(atomic mass of O) = 2(1) + 16 = 18 u. Therefore molar mass of water = 18 g/mol (1 mole of water weighs 18 g). Number of moles = given mass ÷ molar mass = 9 ÷ 18 = 0.5 moles. Number of molecules = moles × Avogadro number = 0.5 × 6.022 × 10²³ = 3.011 × 10²³ water molecules. Now counting atoms: Each H₂O has 2 H atoms and 1 O atom. Hydrogen atoms = 3.011 × 10²³ × 2 = 6.022 × 10²³ H atoms. Oxygen atoms = 3.011 × 10²³ × 1 = 3.011 × 10²³ O atoms. Common mistake in option (a): The molecular mass of water is NOT 9. It is 18. H has atomic mass 1, but water has 2 H atoms (H₂O), and O has atomic mass 16 (not 8 — 8 is the atomic number, not mass). Atomic mass of O = 16, not 8. The Mole concept applies to ALL substances (solids, liquids, gases, ions, atoms, molecules).
Q3. Calcium (Z=20, A=40) and Argon (Z=18, A=40) are called isobars. Which of the following is the most significant implication of this isobar relationship?
- (a) Despite having the same mass number (40 nucleons in each nucleus), calcium and argon are completely different elements with entirely different chemical properties — calcium is a reactive metal (alkaline earth metal) that forms compounds, while argon is an inert noble gas that does not form compounds — because they have different atomic numbers (different numbers of protons and electrons), giving them completely different electron configurations and valencies
- (b) Since calcium and argon are isobars with the same mass number, they have similar chemical properties and can substitute for each other in chemical reactions — isobars behave like isotopes but for different elements
- (c) The isobar relationship means that calcium and argon are found together in nature because they have the same atomic weight — minerals containing calcium often also contain argon in equal proportions
- (d) Isobars cannot actually exist in nature — the mass numbers of different elements are always unique, so calcium (A=40) and argon (A=40) having the same mass number is a theoretical coincidence that has no practical significance
Isobars are atoms of DIFFERENT elements (different Z = different number of protons) that have the SAME mass number A (same total number of nucleons). Calcium: Z=20, A=40, so 20 protons + 20 neutrons. Argon: Z=18, A=40, so 18 protons + 22 neutrons. Same total (40 nucleons) but different composition. The critical point: Chemical properties are determined by electron configuration (number and arrangement of electrons), which equals the number of protons Z in a neutral atom. Calcium has 20 electrons with configuration 2,8,8,2 — 2 electrons in outer shell → valency 2 → reactive metal (forms Ca²⁺, compounds like CaCO₃, CaO, Ca(OH)₂). Argon has 18 electrons with configuration 2,8,8 — completely filled outer shell → valency 0 → noble gas, completely inert, forms no compounds. Despite the same mass number, their chemistries are completely opposite! Contrast with ISOTOPES: Carbon-12 and Carbon-14 — same Z=6, different A. They have the same chemical properties because they have the same electron configuration (both have 6 electrons in 2,4 configuration). The difference: isotopes = same element (same Z) = same chemical behaviour; isobars = different elements (different Z) = different chemical behaviour.
Q4. Dalton's Atomic Theory successfully explained the Law of Conservation of Mass and the Law of Definite Proportions. However, which of the following discoveries most directly invalidated a core postulate of Dalton's theory?
- (a) Lavoisier's discovery that mass is conserved in chemical reactions, which Dalton had not accounted for in his original theory
- (b) Proust's establishment of the Law of Definite Proportions, which showed that compounds always have fixed mass ratios — contradicting Dalton's assumption that atoms could combine in any ratio
- (c) Avogadro's hypothesis that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules — which suggested gas molecules could be diatomic, challenging Dalton's view that elemental gases consist of single atoms
- (d) J.J. Thomson's discovery of the electron (1897), which proved that atoms ARE divisible into sub-atomic particles — directly contradicting Dalton's postulate that atoms are indivisible particles that cannot be created or destroyed
Dalton's most fundamental postulate was that atoms are indivisible — "the smallest particles of matter" that "cannot be created or destroyed." This was the entire conceptual foundation of his atomic theory — the reason it was called "atomic" theory. When J.J. Thomson discovered the electron in 1897 through cathode ray experiments (deflection by electric and magnetic fields showed cathode rays were negatively charged particles lighter than any atom), he proved that atoms contain sub-atomic particles. Atoms ARE divisible — they have internal structure. This directly and completely invalidated Dalton's "indivisible atom" postulate. Subsequently: Goldstein discovered protons (1886, canal rays). Rutherford discovered the nucleus (1911). Chadwick discovered neutrons (1932). Modern quantum mechanics showed electrons don't even move in simple orbits (Heisenberg uncertainty principle). All these discoveries showed atoms have rich internal structure — the complete opposite of Dalton's "solid, indivisible sphere." Option (a) is wrong: Conservation of mass was one of the LAWS that Dalton's theory SUCCESSFULLY explained — not a contradiction. Option (b) is wrong: The Law of Definite Proportions was EXPLAINED by Dalton (fixed atom ratios → fixed mass ratios) — not a contradiction. Option (c): Avogadro's hypothesis did challenge some of Dalton's views about diatomic vs monoatomic gas molecules, but this is a minor issue compared to the fundamental "indivisibility" postulate.
⚡ Quick Revision — Atoms & Molecules
| Topic | Key Facts |
|---|---|
| Laws | Conservation of Mass (Lavoisier): mass neither created nor destroyed. Definite Proportions (Proust): elements in fixed mass ratios. H:O in water always = 1:8. 9g water → 1g H + 8g O. |
| Dalton's Theory (1808) | Atoms indivisible, same element identical, different elements different masses, combine in whole number ratios. Success: explains 2 laws. Failure: atoms ARE divisible (e⁻, p⁺, n discovered); isotopes disprove "same element = same mass." |
| Sub-atomic Particles | Electron (e⁻): Thomson 1897, charge −1, negligible mass. Proton (p⁺): Goldstein 1886, charge +1, mass ~1u. Neutron (n°): Chadwick 1932, charge 0, mass ~1u. Protons + neutrons in nucleus. Electrons in orbits. |
| Atomic Models | Thomson (plum pudding/watermelon): positive sphere + embedded electrons. ❌ Disproved by Rutherford. Rutherford (nuclear/gold foil): tiny dense positive nucleus + electrons in orbits. ❌ Electrons should spiral in. Bohr (shell model): fixed energy shells K/L/M/N (n=1/2/3/4), no radiation in orbit. ✅ Explains stability. |
| Gold Foil Experiment | α-particles at thin gold foil. Observations: most passed (atom mostly empty), some deflected (positive charge in small region), 1/12000 bounced back (tiny dense nucleus). Quote: "15-inch shell at tissue paper." |
| Bohr's Rules | Shell capacity = 2n². K(n=1)=2, L(n=2)=8, M(n=3)=18, N(n=4)=32. Outermost max = 8. Electrons fill shells step by step (inner first). Electrons in discrete orbits don't radiate energy. |
| Key numbers | Atomic No. Z = protons = electrons (neutral). Mass No. A = protons + neutrons. Neutrons = A − Z. Valency = electrons gained/lost/shared to complete octet (8). |
| Isotopes | Same Z, different A. Same chemical properties, different physical. H: Protium (¹H), Deuterium (²H), Tritium (³H). C: ¹²C and ¹⁴C. Cl: ³⁵Cl (75%) and ³⁷Cl (25%) → avg = 35.5. Applications: U-235 (nuclear reactor), Co-60 (cancer), I-131 (goitre), C-14 (dating), Tc-99m (medical imaging). |
| Isobars | Same A, different Z. Different elements, different chemical properties. Ca(Z=20, A=40) and Ar(Z=18, A=40). Completely different chemistry despite same nucleon count. |
| Mole Concept | 1 mole = 6.022 × 10²³ particles (Avogadro's number). Molar mass = molecular/atomic mass in grams. Moles = mass ÷ molar mass. H₂O: mol. mass=18g/mol. CO₂=44. NaCl=58.5. Ca(OH)₂=74. Defined by: atoms in 12g of C-12. |
| Valency |
🚨 5 UPSC Traps — Atoms & Molecules:
Trap 1 — "Rutherford's model proposed electrons in fixed energy shells" → WRONG! Fixed energy shells / discrete orbits is BOHR's contribution (1913) — not Rutherford's. Rutherford's model (1911) had electrons simply revolving around the nucleus in orbits, but gave no explanation for why they don't spiral inward. Bohr (1913) solved this by proposing specific allowed orbits with discrete energies. Pattern Q above directly tests this distinction — Statement 3 attributes Bohr's idea to Rutherford and is WRONG.
Trap 2 — "Isotopes have similar chemical AND physical properties" → WRONG! Isotopes have similar CHEMICAL properties but DIFFERENT physical properties. Since chemical properties depend on electron configuration (same for isotopes — same Z), isotopes react similarly. But physical properties (mass, density, radioactivity, melting/boiling point) differ because the atoms have different masses. ¹²C (stable) and ¹⁴C (radioactive) — same chemistry, very different physics.
Trap 3 — "The atomic mass of Oxygen is 8" → WRONG! 8 is the ATOMIC NUMBER of Oxygen (number of protons). The ATOMIC MASS of Oxygen is 16 u. This is the most common calculation error. Molecular mass of water: H₂O = 2(1) + 16 = 18, NOT 2(1)+8=10. Always remember: atomic number Z=8 for O, atomic mass=16 for O. The confusion arises because there are 8 protons AND 8 neutrons in ¹⁶O → total 16.
Trap 4 — "Isobars have similar chemical properties like isotopes" → WRONG! Isobars have the SAME MASS NUMBER but COMPLETELY DIFFERENT chemical properties — because they are different elements with different atomic numbers (different electron configurations). Calcium (Z=20, reactive metal, valency 2) and Argon (Z=18, inert noble gas, valency 0) are isobars — their chemistry is utterly different. Isotopes share chemical properties; isobars do NOT.
Trap 5 — "Thomson discovered the proton; Goldstein discovered the electron" → WRONG! (Discovery roles confused) Thomson discovered the ELECTRON (1897) via cathode ray tube experiments. Goldstein discovered the PROTON (1886) via canal rays (positively charged rays moving toward the cathode). Chadwick discovered the NEUTRON (1932). These are often swapped in UPSC traps — remember the order: E-T-G-P (Thomson → Electron; Goldstein → Proton; Chadwick → Neutron) or "Thomson electrons, Goldstein Protons."
Trap 1 — "Rutherford's model proposed electrons in fixed energy shells" → WRONG! Fixed energy shells / discrete orbits is BOHR's contribution (1913) — not Rutherford's. Rutherford's model (1911) had electrons simply revolving around the nucleus in orbits, but gave no explanation for why they don't spiral inward. Bohr (1913) solved this by proposing specific allowed orbits with discrete energies. Pattern Q above directly tests this distinction — Statement 3 attributes Bohr's idea to Rutherford and is WRONG.
Trap 2 — "Isotopes have similar chemical AND physical properties" → WRONG! Isotopes have similar CHEMICAL properties but DIFFERENT physical properties. Since chemical properties depend on electron configuration (same for isotopes — same Z), isotopes react similarly. But physical properties (mass, density, radioactivity, melting/boiling point) differ because the atoms have different masses. ¹²C (stable) and ¹⁴C (radioactive) — same chemistry, very different physics.
Trap 3 — "The atomic mass of Oxygen is 8" → WRONG! 8 is the ATOMIC NUMBER of Oxygen (number of protons). The ATOMIC MASS of Oxygen is 16 u. This is the most common calculation error. Molecular mass of water: H₂O = 2(1) + 16 = 18, NOT 2(1)+8=10. Always remember: atomic number Z=8 for O, atomic mass=16 for O. The confusion arises because there are 8 protons AND 8 neutrons in ¹⁶O → total 16.
Trap 4 — "Isobars have similar chemical properties like isotopes" → WRONG! Isobars have the SAME MASS NUMBER but COMPLETELY DIFFERENT chemical properties — because they are different elements with different atomic numbers (different electron configurations). Calcium (Z=20, reactive metal, valency 2) and Argon (Z=18, inert noble gas, valency 0) are isobars — their chemistry is utterly different. Isotopes share chemical properties; isobars do NOT.
Trap 5 — "Thomson discovered the proton; Goldstein discovered the electron" → WRONG! (Discovery roles confused) Thomson discovered the ELECTRON (1897) via cathode ray tube experiments. Goldstein discovered the PROTON (1886) via canal rays (positively charged rays moving toward the cathode). Chadwick discovered the NEUTRON (1932). These are often swapped in UPSC traps — remember the order: E-T-G-P (Thomson → Electron; Goldstein → Proton; Chadwick → Neutron) or "Thomson electrons, Goldstein Protons."
