Chemical Bonding

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Chemical bonding is the force of attraction that holds atoms together in molecules and compounds. The type of bond formed determines the physical and chemical properties of the substance — melting point, boiling point, electrical conductivity, solubility, and reactivity all stem from the nature of the chemical bond. Understanding chemical bonding is essential for predicting molecular geometry, polarity, and reactivity.

Key Definitions:

• Ionic bond: Electrostatic attraction between oppositely charged ions formed by electron transfer
• Covalent bond: Shared pair of electrons between two atoms
• Coordinate bond (dative bond): Shared electron pair where both electrons come from one atom
• Hydrogen bond: Attraction between H atom bonded to F, O, or N and a lone pair of F, O, or N
• Van der Waals forces: Weak intermolecular attractions (London dispersion forces)

Lewis Structure Rules:

1. Count valence electrons
2. Draw skeleton structure
3. Put electrons as dots around atoms (octet rule, except H = 2, B = 6)
4. If electron-deficient, form double/triple bonds
5. Formal charge = valence electrons $-$ non-bonding electrons $- \frac{1}{2} \times$ bonding electrons

VSEPR Molecular Geometry:

Bond pairs	Lone pairs	Shape	Example
2	0	Linear	CO₂, BeCl₂
3	0	Trigonal planar	BF₃
4	0	Tetrahedral	CH₄
3	1	Trigonal pyramidal	NH₃
2	2	Bent/V-shaped	H₂O
3	2	T-shaped	ClF₃

⚡ Exam Tip: In NEET, always check formal charges — a Lewis structure with formal charges closer to zero on all atoms is the correct one. Also remember: resonance structures differ only in electron distribution, not atomic positions. The actual molecule is a hybrid.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding and problem-solving practice.

Hybridisation:

$s p^3$ hybridisation: one s + three p orbitals mix to form four equivalent $sp^3$ orbitals (109.5° bond angle, tetrahedral geometry)

$s p^2$ hybridisation: one s + two p orbitals form three $sp^2$ orbitals (120° bond angle, trigonal planar). Leftover p orbital forms pi bond.

$sp$ hybridisation: one s + one p form two sp orbitals (180° bond angle, linear). Two leftover p orbitals form two pi bonds (triple bond).

Fajan’s Rules — Covalent Character in Ionic Bonds: Larger cation + smaller anion $\Rightarrow$ greater covalent character Higher charge on cation $\Rightarrow$ greater covalent character Cation with electronic configuration $(n-1)d^n ns^0$ (pseudo noble gas) $\Rightarrow$ greater covalent character

Bond Parameters:

• Bond length: measured in picometres (pm); single bond > double bond > triple bond in length
• Bond angle: angle between lines joining centre of atom to surrounding atoms
• Bond enthalpy: energy required to break one mole of bonds (kJ mol⁻¹)
• Bond order: number of bonds between two atoms (Bond order 3 = triple bond, Bond order 2 = double bond)

Molecular Orbital Theory (MOT):

Electron configuration for N₂ (14 electrons): $\sigma1s^2, \sigma^*1s^2, \sigma2s^2, \sigma^*2s^2, \pi2p_x^2 = \pi2p_y^2, \sigma2p_z^2$

Bond Order = $\frac{1}{2}(N_b - N_a) = \frac{1}{2}(10 - 4) = 3$ (triple bond)

For O₂ (16 electrons): $\sigma1s^2, \sigma^*1s^2, \sigma2s^2, \sigma^*2s^2, \sigma2p_z^2, \pi2p_x^2 = \pi2p_y^2, \pi^*2p_x^1 = \pi^*2p_y^1$

Bond Order = $\frac{1}{2}(8 - 6) = 1$ (paramagnetic — two unpaired electrons in $\pi^*$ orbitals, matches experimental data)

Dipole Moment: $\mu = q \times d$ (charge × distance between centres) Units: Debye (D); 1 D = $3.335 \times 10^{-30}$ C·m

Vector sum of individual bond dipoles gives molecular dipole. For example, NH₃ has a net dipole moment because the three N-H bond dipoles don’t cancel completely (lone pair contributes to net dipole pointing toward lone pair).

⚡ NEET-Specific Tip: Questions on MOT for diatomic molecules like B₂, C₂, N₂ are frequently asked. Remember the energy order: $\sigma2p < \pi2p < \sigma2p_z < \pi^*2p < \sigma^*2p_z$. B₂ has 10 electrons, is paramagnetic with 2 unpaired electrons — this is a classic NEET question.

Common Student Mistakes:

• Confusing bond angle values ($sp^3$ = 109.5°, $sp^2$ = 120°, $sp$ = 180°)
• Forgetting that lone pairs occupy more space than bonding pairs (they compress bond angles)
• Memorising MOT without understanding the electron filling order

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory with derivations and exam pattern analysis.

Valence Bond Theory (VBT):

VBT explains bond formation through overlap of atomic orbitals. Types of orbital overlap:

• Sigma ($\sigma$) bond: Head-on overlap along the bond axis — strongest bond type
• Pi ($\pi$) bond: Sideways overlap above and below the bond plane — weaker than $\sigma$

Single bond = one $\sigma$ bond; Double bond = one $\sigma$ + one $\pi$ bond; Triple bond = one $\sigma$ + two $\pi$ bonds

Hybridisation in Polyatomic Molecules:

• BeCl₂: $sp$ hybridisation, linear (no lone pair on Be)
• BF₃: $sp^2$ hybridisation, trigonal planar (no lone pair on B)
• PCl₅: $sp^3d$ hybridisation, trigonal bipyramidal (axial and equatorial bonds: 90° and 120°)
• SF₆: $sp^3d^2$ hybridisation, octahedral (all bonds 90°)
• XeF₂: $sp^3d$ hybridisation, linear (three lone pairs in equatorial positions)
• IF₇: $sp^3d^3$, pentagonal bipyramidal

VSEPR Limitations and Bent’s Rule: Bent’s Rule: More electronegative atom demands more s-character in hybrid orbital used for bonding. In CH₂Cl₂, the C-Cl bonds have more s-character (closer to 180°) than C-H bonds because Cl is more electronegative than H.

Hydrogen Bonding:

Intramolecular H-bonding: Present within the same molecule (e.g., o-nitrophenol — prevents intermolecular H-bonding and makes it steam-volatile)

Intermolecular H-bonding: Between different molecules (e.g., water — explains high boiling point 100°C, high surface tension, ice having lower density than water)

H-bond energy: 10-40 kJ mol⁻¹ (much weaker than covalent bonds ~200-400 kJ mol⁻¹ but stronger than van der Waals ~0.1-10 kJ mol⁻¹)

Motivation for Bonding Theories: MOT was developed because VSEPR fails to explain: (1) Why O₂ is paramagnetic despite all electrons paired in Lewis structure, (2) Delocalised electrons in benzene, (3) Bond order fractional numbers in resonance species like benzene (C₆H₆ bond order 1.5 for each C-C bond).

NEET Previous Year Patterns (2019-2024):

• 2019: MOT electron configuration for O₂²⁻ (2 marks)
• 2020: Hybridisation of P in PCl₅ and S in SF₆ (2 marks)
• 2021: VSEPR shape prediction for XeF₄ (square planar, 4 lone pairs) (2 marks)
• 2022: Dipole moment comparison CH₃Cl vs CHCl₃ vs CCl₄ (2 marks)
• 2023: Bond order calculation for O₂⁺ (1.5) and comparison of bond dissociation energy (2 marks)
• 2024: Resonance structures for nitrate ion (NO₃⁻) with formal charges (3 marks)

⚡ Advanced Tip: For formal charge calculations in resonance, remember: total formal charge should equal the overall charge of the species. In NO₃⁻ (12 total valence electrons), the correct resonance structure has one double-bonded O and two single-bonded O atoms with formal charges: double-bonded O = 0, single-bonded O = -1 each, N = +1. Net charge = -1, total valence electrons check: 6 + 5 + 6(3) = 23, but minus the lone electrons lost for bonds: each N-O single bond loses 1 electron from N and 1 from O, so 23 - 4 = 19 electrons in the structure + 1 for negative charge = 20… verify carefully during exam.

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📊 NEET UG Exam Essentials

Detail	Value
Questions	200 (180 mandatory + 10 optional)
Time	3h 20min
Marks	720
Section	Physics (50), Chemistry (50), Biology (100)
Negative	−1 for wrong answer
Qualifying	50th percentile (general category)

🎯 High-Yield Topics for NEET UG

• Human Physiology — 18 marks
• Genetics & Evolution — 16 marks
• Ecology & Environment — 12 marks
• Organic Chemistry (Reactions) — 15 marks
• Electrodynamics (Physics) — 18 marks
• Chemical Equilibrium — 10 marks

📝 Previous Year Question Patterns

• Q: “A particle moves in a circle…” [2024 Physics — 2 marks]
• Q: “Identify the incorrect statement about DNA…” [2024 Biology — 4 marks]
• Q: “The major product ofFriedel-Crafts acylation is…” [2024 Chemistry — 3 marks]

💡 Pro Tips

• NCERT Biology is the single most important resource — 80%+ questions are from NCERT lines
• Focus on Human Physiology, Genetics, and Ecology — together they make ~40% of Biology
• In Physics, master Electrostatics + Current Electricity + Magnetism (combined ~20%)
• Organic Chemistry: learn named reactions with mechanisms — they repeat across years

🔗 Official Resources

• NTA Official
• NEET Syllabus PDF

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