Introduction
Valency, Chemical Bonding, Redox Reactions, Electrolysis and Electrochemical Series form the core foundation of Chemistry and carry 10–15 marks across UPSC Prelims, SSC CGL/CHSL/MTS, RRB NTPC/ALP, NEET, JEE Main in General Science/Chemistry sections. This integrated topic tests conceptual understanding + numerical application through direct questions on bond types, oxidation numbers, electrolysis products and reactivity series.
Why This Topic is High-Scoring:
· Valency = Easy 1-mark questions (Group valencies)
· Chemical Bonding = 3–4 marks (Ionic vs Covalent, VSEPR shapes)
· Redox Reactions = 4–5 marks (Balancing, oxidation number calculation)
· Electrolysis = 3 marks (Cathode/anode products, Faraday's laws)
· Electrochemical Series = 2 marks (Displacement, reactivity order)
This blog covers everything from basic valency rules to advanced electrochemical series applications with:
· Complete classification of chemical bonds (Ionic, Covalent, Coordinate, Metallic)
· Modern definitions of oxidation/reduction (LEO GER)
· Step-by-step redox balancing (both methods)
· Electrolysis rules with preferential discharge series
· Electrochemical series table with exam applications
· 25 MCQ PYQ patterns
Perfect for non-IIT students preparing UPSC/SSC/NEET who need clear concepts + exam shortcuts without complex theory.
Valency
Definition: Valency is the combining capacity of an atom determined by number of electrons it can lose, gain or share to achieve stable octet configuration (8 electrons in valence shell).
Types of Valency:
1. Electrovalency (Ionic) – Electrons lost/gained
2. Covalency – Electrons shared
3. Variable Valency – Transition metals (Fe²⁺/Fe³⁺)
Valency Rules Table:
|
Group |
Valency |
Examples |
|
Group 1 (IA) |
+1 |
Na⁺, Li⁺ |
|
Group 2 (IIA) |
+2 |
Mg²⁺, Ca²⁺ |
|
Group 13 (IIIA) |
+3 |
Al³⁺ |
|
Group 14 (IVA) |
+4/-4 |
C (+4), Si (+4) |
|
Group 15 (VA) |
-3/+5 |
N (-3/+5), P (-3/+5) |
|
Group 16 (VIA) |
-2/+6 |
O (-2), S (-2/+6) |
|
Group 17 (VIIA) |
-1 |
F⁻, Cl⁻ |
|
Group 18 (VIIIA) |
0 |
Noble gases (stable) |
Exam
Shortcut: Variable
valency = Transition metals (d-block).
PARLIAMENTARY COMMITTEES OF INDIA COMPLETE NOTES FOR UPSC, SSC CGL, RRB NTPC!
Chemical Bonding
Why Chemical Bonding Occurs: Atoms achieve stable electronic configuration (noble gas: 8 valence electrons).
Four Main Types:
1. Ionic Bond (Electrovalent)
2. Covalent Bond
3. Coordinate (Dative) Bond
4. Metallic Bond
Bond Formation Decision Table:
|
Elements |
Bond Type |
Reason |
|
Metal + Non-metal |
Ionic |
Large electronegativity difference |
|
Non-metal + Non-metal |
Covalent |
Similar electronegativity |
|
Transition metals |
Metallic |
Delocalized electrons |
|
Lone pair donor |
Coordinate |
Complete electron pair sharing |
Hydrosphere Relief of Ocean Floor - Continental Shelf, Slope, Complete Notes.
Ionic Bond
Definition: Complete transfer of electrons from metal to non-metal forming oppositely charged ions held by electrostatic attraction.
Formation Steps:
Na (2,8,1) + Cl (2,8,7)
↓
Na → Na⁺ + e⁻ (loses 1e⁻)
Cl + e⁻ → Cl⁻ (gains 1e⁻)
↓
Na⁺Cl⁻ (NaCl) (ionic bond)
Properties of Ionic Compounds:
|
Property |
Characteristic |
|
High MP/BP |
Strong electrostatic forces |
|
Soluble in water |
Hydration energy > lattice energy |
|
Conduct electricity |
Ions free in molten/aqueous state |
|
Hard & brittle |
Cation-anion alignment |
Examples: NaCl, MgO, CaCl₂, KBr.
Learn Powers, functions, and Articles of Prime Minister & Council of Ministers with examples.
Covalent Bond
Definition: Sharing of electron pairs between atoms (no complete transfer).
Types Based on Shared Pairs:
Single Bond: 1 electron pair (H-H, Cl-Cl)
Double Bond: 2 electron pairs (O=O, C=C)
Triple Bond: 3 electron pairs (N≡N)
Polar vs Non-polar Covalent:
|
Type |
Electronegativity Difference |
Examples |
|
Non-polar |
0 (identical atoms) |
H₂, O₂, N₂, Cl₂ |
|
Polar |
0.4–1.7 |
HCl, H₂O, NH₃ |
VSEPR Theory (Shape Prediction):
Linear: BeCl₂, CO₂ (180°)
Trigonal Planar: BF₃ (120°)
Tetrahedral: CH₄ (109°28')
Trigonal Pyramidal: NH₃ (107°)
Bent/V-shaped: H₂O (104.5°)
WORK ENERGY POWER – COMPLETE NOTES!
Coordinate Bond & Metallic Bonding
Coordinate (Dative) Bond: One atom provides both electrons of shared pair.
Examples: NH₄⁺ (N→H), H₃O⁺ (O→H)
Metallic Bond: Sea of delocalized electrons between metal cations.
Properties: Conductivity, Malleability, Ductility, Lustre
Examples: Cu, Ag, Fe, Al
Oxidation & Reduction
Modern Definition (Electron Transfer):
OXIDATION = Loss of electrons / Increase in oxidation number
REDUCTION = Gain of electrons / Decrease in oxidation number
Traditional vs Modern:
|
Traditional |
Modern |
|
Add oxygen |
Lose e⁻ |
|
Remove hydrogen |
Gain e⁻ |
Mnemonic: LEO GER (Loss Electrons Oxidation, Gain Electrons Reduction)
Oxidation Number Rules (Exam Essential):
1. Uncombined elements = 0
2. H = +1 (except metal hydrides -1)
3. O = -2 (except peroxides -1, OF₂ +2)
4. Group 1 = +1, Group 2 = +2
5. Fluorine = -1 (always)
MONEY & BANKING – COMPLETE EXAM NOTES WITH MCQs.
Redox Reactions
Method 1: Oxidation Number Method
Step 1: Assign oxidation numbers
Step 2: Identify increase/decrease
Step 3: Balance electron transfer
Step 4: Balance other atoms
Step 5: Balance charge with H⁺/OH⁻
Example: MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺
Method 2: Half-Reaction Method
Oxidation Half: Fe²⁺ → Fe³⁺ + e⁻
Reduction Half: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
Combine: Multiply to balance electrons
Electrolysis – Faraday's Laws
Faraday's
First Law:
Mass ∝ Quantity
of electricity
m = Z × It
Where Z = Electrochemical equivalent
Faraday's Second Law:
Same charge deposits equivalent masses
H = 1, O = 8, Al = 9, Ag = 108
Electrolysis Rules (Electrochemical Series):
CATHODE (-): Lower in series
discharges (easier reduction)
ANODE (+): Higher in series discharges (easier oxidation)
Preferential Discharge Series:
Cations: K⁺ < Na⁺ < Ca²⁺ < Mg²⁺ < Al³⁺ < Zn²⁺ < Fe²⁺ < Cu²⁺ < Ag⁺ < H⁺
Anions: SO₄²⁻ < NO₃⁻ < Cl⁻ < OH⁻ < O²⁻
Examples:
CuSO₄ + Pt electrodes → Cu (cathode), O₂ (anode)
NaCl (aq) → H₂ (cathode), Cl₂ (anode)
Electrochemical Series – Reactivity Order
Definition: Metals/non-metals arranged by standard reduction potential (E°).
Key Points:
Top (Strong Reducing Agents): K, Na, Ca, Mg (E° = -2 to -3V)Middle: Zn, Fe, Sn, Pb, HBottom (Weak): Cu, Ag, Au (E° = +0.3 to +1.5V)Uses:
1. Reactivity: Top more reactive
2. Displacement: Zn + CuSO₄ → ZnSO₄ + Cu
3. Electrolysis: Lower discharges first
4. EMF of cell: E_cell = E_cathode - E_anode
MCQs
Q1.
Valency of an element is determined by:
(A) Atomic number
(B) Number of valence
electrons
(C) Atomic mass
(D) Number of neutrons
Ans: B
Explanation:
Valency = electrons lost/gained/shared for octet. [SSC CGL]
Q2. Which
bond forms between metal and non-metal?
(A) Covalent
(B) Ionic
(C) Coordinate
(D) Metallic
Ans: B
Explanation:
Large electronegativity
difference → electron transfer. [RRB NTPC]
Q3. Which
has highest melting point?
(A) Covalent compound
(B) Ionic compound
(C) Molecular solid
(D) Metallic
Ans: B
Explanation:
Ionic =
strong electrostatic forces. [SSC CHSL]
Q4. Shape
of NH₃ molecule (VSEPR):
(A) Tetrahedral
(B) Trigonal pyramidal
(C) Linear
(D) Bent
Ans: B
Explanation:
3 bond pairs + 1 lone
pair → trigonal pyramidal. [NEET]
Q5. Which
shows variable valency?
(A) Na
(B) Al
(C) Fe (Fe²⁺/Fe³⁺)
(D) Carbon
Ans: C
Explanation:
Transition metals
(d-block). [JEE Main]
Q6. Polar
covalent bond example:
(A) H₂
(B) HCl
(C) Cl₂
(D) N₂
Ans: B
Explanation:
Electronegativity
difference 0.4–1.7. [SSC MTS]
Q7.
Coordinate bond in:
(A) NaCl
(B) NH₄⁺
(C) CH₄
(D) MgO
Ans: B
Explanation:
N donates lone pair to H⁺. [NEET]
Q8.
Metallic bond explains:
(A) High solubility
(B) Malleability
(C) Low conductivity
(D) High volatility
Ans: B
Explanation:
Delocalized electrons
between cations. [RRB ALP]
Q9. Modern
definition of oxidation:
(A) Gain of oxygen
(B) Loss of electrons
(C) Gain of hydrogen
(D) Loss of oxygen
Ans: B
Explanation:
LEO
(Loss Electrons Oxidation). [UPSC CDS]
Q10.
Oxidation number of Cr in K₂Cr₂O₇:
(A) +3
(B) +4
(C) +6
(D) +7
Ans: C
Explanation:
2(+1) + 2x + 7(-2) = 0 → x = +6. [SSC CGL]
Q11.
Strongest reducing agent:
(A) F₂
(B) Li
(C) Cl₂
(D) O₂
Ans: B
Explanation: Most
negative E° (top of electrochemical series). [NEET]
Q12. In
redox reaction, oxidising agent:
(A) Loses electrons
(B) Gets reduced
(C) Increases ON
(D) Gains hydrogen
Ans: B
Explanation:
Oxidising agent causes
oxidation (itself gets reduced). [JEE Main]
Q13. Which
is redox reaction?
(A) NaCl → Na⁺ + Cl⁻
(B) Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
(C) CaCO₃ →
CaO + CO₂
(D) NH₄Cl +
NaOH → NaCl + NH₃ + H₂O
Ans:B
Explanation:
Cu oxidised, Ag⁺ reduced. [SSC CHSL]
Q14.
Oxidation number of S in H₂SO₄:
(A) +2
(B) +4
(C) +6
(D) +8
Ans: C
Explanation:
2(+1) + x + 4(-2) = 0 → x = +6. [RRB NTPC]
Q15.
Substance acting as both oxidising & reducing agent:
(A) KMnO₄
(B) H₂O₂
(C) K₂Cr₂O₇
(D) HNO₃
Ans: B
Explanation:
H₂O₂ can lose OR gain electrons. [NEET]
Q16.
Cathode product in dilute NaCl electrolysis:
(A) Na
(B) H₂
(C) Cl₂
(D) NaOH
Ans: B
Explanation:
H⁺ easier than Na⁺ (electrochemical series). [SSC CGL]
Q17. Anode
product in CuSO₄ electrolysis (Pt electrodes):
(A) Cu
(B) O₂
(C) SO₂
(D) CuO
Ans: B
Explanation:
OH⁻ easier than SO₄²⁻. [JEE Main]
Q18.
Faraday's First Law:
(A) m ∝ 1/It
(B) m ∝ It
(C) m ∝ Z
(D) m ∝ 1/Z
Ans: B
Explanation:
Mass ∝ charge passed. [RRB ALP]
Q19. In electrolysis,
preferential discharge follows:
(A) Atomic number
(B) Electrochemical
series
(C) Valency
(D) Concentration only
Ans: B
Explanation:
Lower in series at
cathode. [NEET]
Q20.
Molten NaCl electrolysis gives:
(A) Na (cathode), Cl₂
(anode)
(B) H₂
(cathode), Cl₂ (anode)
(C) Na (cathode), O₂
(anode)
(D) H₂
(cathode), O₂ (anode)
Ans: A
Explanation:
No water
→ Na metal. [SSC MTS]
Q21. Which
displaces H₂ from acid?
(A) Cu
(B) Ag
(C) Zn
(D) Au
Ans: C
Explanation:
Zn > H
in series. [SSC CGL]
Q22.
Strongest oxidising agent:
(A) Li
(B) F₂
(C) Na
(D) Mg
Ans: B
Explanation:
Most positive E°
(bottom of series). [NEET]
Q23. Zn +
CuSO₄ → ?
(A) No reaction
(B) ZnSO₄ + Cu
(C) ZnSO₄ + CuSO₄
(D) Zn + Cu
Ans: B
Explanation:
Zn > Cu
(Zn displaces Cu). [JEE Main]
Q24.
Electrochemical series arranged by:
(A) Atomic radius
(B) Standard reduction
potential
(C) Ionisation energy
(D) Electronegativity
Ans: B
Explanation:
E° values
determine reactivity. [RRB NTPC]
Q25. Metal
with most negative E° (strongest reducing agent):
(A) Cu (+0.34V)
(B) K (-2.92V)
(C) Ag (+0.80V)
(D) Fe (-0.44V)
Ans: B
Explanation:
More negative E° =
stronger reducing agent. [NEET]
FAQs
Q1. Modern definition of oxidation?
Loss of electrons / increase in oxidation number.
Q2. Which bond has highest MP/BP?
Ionic bond (strong electrostatic forces).
Q3. Electrochemical series arranged
by?
Standard reduction potential (E° values).
Q4. Cathode product in NaCl (aq)
electrolysis?
H₂
gas (H⁺ easier than Na⁺).
Q5. Variable valency shown by?
Transition metals (d-block elements).
Q6. Polar covalent bond example?
HCl, H₂O
(electronegativity difference 0.4–1.7).
Conclusion:
Valency → Bonding → Redox → Electrolysis → Electrochemical Series forms an integrated Chemistry chain that guarantees 10–15 marks in competitive exams. Master these 5 pillars and Chemistry becomes your scoring subject.
Exam-Ready Summary (Memorise This Table):
|
Topic |
Key Formula/Fact |
|
Valency |
Groups 1-2: +1,+2; 13-17: variable |
|
Ionic Bond |
Metal+Non-metal, High MP/BP |
|
Covalent |
Non-metal+Non-metal, VSEPR shapes |
|
Redox |
LEO GER (Lose/Gain electrons) |
|
Electrolysis |
Cathode: Lower series, Anode: Higher |
|
Electrochemical |
Zn > Fe > H > Cu > Ag reactivity |
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