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CA03 — Chemical Classification & Reactions

✈ Chemistry – CA03  ·  AFCAT General Awareness AFCAT Level ★ High Priority

The periodic table organises all elements by atomic number, revealing predictable patterns. Types of chemical reactions classify how substances transform. AFCAT tests periodic trends (which direction increases/decreases) and reaction types with real-life examples directly — expect 1–2 questions.

📌 AFCAT Focus: Modern periodic law (atomic number, not mass); atomic radius decreases across a period; ionisation energy and electronegativity increase across a period; fluorine = most electronegative; displacement reaction (more reactive element displaces less reactive); OIL RIG (Oxidation = loss, Reduction = gain of electrons).
PART 1 — PERIODIC TABLE

1. Modern Periodic Law & Periodic Trends

Fig. 1 — Periodic Trends: Direction of Increase Across Period and Down Group
PERIODIC TRENDS — Across Period (→) and Down Group (↓) Property Across Period → Down Group ↓ Key fact Atomic Radius (size of atom) DECREASES → More protons pull e⁻ inward INCREASES ↓ New shells added Largest at bottom-left Ionisation Energy INCREASES → Smaller atom; e⁻ held tighter DECREASES ↓ Larger atom; e⁻ farther away He = highest Cs = lowest Electro- negativity INCREASES → More nuclear attraction DECREASES ↓ Shielding effect increases F = MOST electronegative! Metallic Character DECREASES → Right = non-metals (halogens) INCREASES ↓ Easier to lose electrons K = most reactive metal
💡 Key Groups to Know for AFCAT: Group 1 = Alkali metals (Li, Na, K — highly reactive, store in kerosene); Group 17 = Halogens (F, Cl, Br — most reactive non-metals, 7 valence electrons); Group 18 = Noble gases (He, Ne, Ar — completely filled, inert). The Modern Periodic Law: properties are a periodic function of atomic number (not atomic mass as Mendeleev proposed).
PART 2 — TYPES OF CHEMICAL REACTIONS

2. Six Types of Chemical Reactions

Fig. 2 — Six Reaction Types with Equations — Each Has a Distinct Pattern
SIX TYPES OF CHEMICAL REACTIONS Type Pattern & Definition Example 1. COMBINATION A + B → AB Two or more substances combine to form a single product. 2H₂ + O₂ → 2H₂O CaO + H₂O → Ca(OH)₂ 2. DECOMPOSITION AB → A + B One compound breaks down into two or more simpler ones. 2H₂O → 2H₂ + O₂ ↑ CaCO₃ → CaO + CO₂ ↑ 3. DISPLACEMENT A + BC → AC + B More reactive element displaces a less reactive one from its salt. Zn + H₂SO₄ → ZnSO₄ + H₂↑ Fe + CuSO₄ → FeSO₄ + Cu 4. DOUBLE DISP. AB + CD → AD + CB Ions in two compounds exchange partners; often forms precipitate. AgNO₃ + NaCl → AgCl↓ + NaNO₃ 5. OXIDATION (loss of electrons) Gain of O, or loss of H or e⁻. Substance is called reducing agent. 2Mg + O₂ → 2MgO Na → Na⁺ + e⁻ 6. REDUCTION (gain of electrons) Loss of O, or gain of H or e⁻. Substance is called oxidising agent. CuO + H₂ → Cu + H₂O Cl₂ + 2e⁻ → 2Cl⁻
💡 OIL RIG — The Most Important Redox Mnemonic:
Oxidation Is Loss (of electrons)    |    Reduction Is Gain (of electrons)
In a redox reaction: the substance oxidised = reducing agent (gives electrons); the substance reduced = oxidising agent (takes electrons). Both always happen together — hence "redox" (reduction-oxidation).

📝 AFCAT PYQs — Reactions & Periodic Table

Q1. The modern periodic table is based on which property of elements? AFCAT PYQ
(a) Atomic mass(b) Atomic number(c) Valency(d) Melting point
✔ Answer: (b) Atomic number
The Modern Periodic Law (Moseley, 1913): properties of elements are a periodic function of their atomic number (Z). Mendeleev used atomic mass — this created anomalies (e.g., Ar/K pair). Moseley's X-ray experiments showed atomic number is the fundamental quantity. This distinction is directly tested in AFCAT.
Q2. Fe + CuSO₄ → FeSO₄ + Cu is an example of: AFCAT PYQ
(a) Combination reaction(b) Decomposition reaction(c) Displacement reaction(d) Double displacement
✔ Answer: (c) Displacement reaction
Iron is more reactive than copper in the reactivity series (Fe > Cu). Therefore, Fe displaces Cu from CuSO₄ solution — a classic single displacement reaction. This is also a redox reaction: Fe is oxidised (loses electrons to become Fe²⁺); Cu²⁺ is reduced (gains electrons to become Cu metal). The blue CuSO₄ solution turns pale green as FeSO₄ forms.
Q3. Which element has the highest electronegativity? AFCAT PYQ
(a) Oxygen(b) Chlorine(c) Nitrogen(d) Fluorine
✔ Answer: (d) Fluorine
Fluorine (F) is the most electronegative element in the periodic table (Pauling scale = 3.98 ≈ 4.0). It is at the top-right of the p-block — maximum nuclear charge, minimum shielding, smallest atomic radius. It is also the strongest oxidising agent and most reactive non-metal. This is one of the most repeated AFCAT chemistry facts.

🧠 Quick Memory Chart — CA03

📈 Periodic Trends
  • Atomic radius: ↓ across; ↑ down group
  • Ionisation energy: ↑ across; ↓ down
  • Electronegativity: ↑ across; ↓ down
  • F = most electroneg. (4.0)
  • Cs = most reactive metal (bottom-left)
⚖ Key Groups
  • Group 1: Alkali metals (Li→Fr)
  • Group 17: Halogens (F→At)
  • Group 18: Noble/inert gases
  • Groups 3–12: Transition metals
  • Modern law: atomic number (not mass)
⚛ Reaction Types
  • Combination: A+B→AB
  • Decomposition: AB→A+B
  • Displacement: reactive displaces less
  • Double disp: ions exchange (precipitate)
  • OIL RIG: Oxidation=Loss; Reduction=Gain

📝 Practice Exercise

E1. 2H₂O → 2H₂ + O₂ is an example of:
(a) Combination(b) Decomposition(c) Displacement(d) Double displacement
E2. Across a period (left to right), atomic radius:
(a) Increases(b) Decreases(c) Stays same(d) First increases then decreases
E3. In the reaction CuO + H₂ → Cu + H₂O, hydrogen is acting as:
(a) Oxidising agent(b) Reducing agent(c) Catalyst(d) Neither
Answers:
E1 → (b) Decomposition [one compound splits into simpler substances]  |  E2 → (b) Decreases [increasing nuclear charge with same shell pulls electrons inward; smaller radius]  |  E3 → (b) Reducing agent [H₂ reduces CuO by removing oxygen from it; H₂ is itself oxidised to H₂O]
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