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Chemical Bonding and Molecular Structure
Chemistry · Class 11NCERT pp. 100–1322 sets × 25 questions
Key facts and formulas for Chemical Bonding and Molecular Structure, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.
Key facts for this chapter
| Idea | Rule or fact |
|---|
| Formal charge | FC = valence electrons − lone-pair electrons − ½ bonding electrons |
| Octet rule exceptions | Incomplete octet (BeCl₂, BF₃), expanded octet (SF₆, PCl₅), odd electron (NO, NO₂) |
| Dipole moment | μ = q × d; 1 D = 3.336 × 10⁻³⁰ C m; symmetric molecules have zero dipole |
| VSEPR order of repulsion | lone pair–lone pair > lone pair–bond pair > bond pair–bond pair |
| Shapes by pairs | AB₃E pyramidal, AB₄E see-saw, AB₃E₂ T-shaped, AB₂E₃ linear, AB₄E₂ square planar, AB₅E square pyramidal |
| Bond angles | BF₃ 120°, CH₄ 109.5°, NH₃ 107°, H₂O 104.5° |
| Hybridisation | 2 domains sp, 3 sp², 4 sp³, 5 sp³d, 6 sp³d²; s character 50 %, 33 %, 25 % |
| Sigma and pi bonds | Every bond has one σ; a double bond adds one π, a triple bond adds two π |
| Molecular orbital bond order | Bond order = (Nb − Na)/2; higher order means shorter, stronger bond |
| Magnetic behaviour | Unpaired electrons give paramagnetic; all paired gives diamagnetic (O₂, B₂ para; N₂, C₂ dia) |
| Hydrogen bonding | Needs H on N, O or F; intermolecular raises boiling point, intramolecular (o-nitrophenol) makes it volatile |
| Fajans rules | Small, highly charged cation and large anion give more covalent character |
Three sample questions
Q1. An element has five dots in its Lewis symbol. Its common valence is
- 5 or 3
- 5 only
- 3 only
- 8 − 3 = 5 only
Hint and solution
Hint: Group valence is either the number of dots or eight minus the number of dots.
- valence electrons = dots = 5 — The number of dots in a Lewis symbol equals the number of valence electrons, here 5.
- valence = dots or (8 − dots) — Group valence is usually the number of dots or eight minus that number.
- 8 − 5 = 3 — The atom can show valence 5 by sharing its five electrons, or valence 3 by gaining three to complete an octet.
- Answer: 5 or 3 — So the common valence is 5 or 3. Option A.
Answer: A. Group valence is the dot count or 8 − dots, so 5 or 3.
Q2. The number of valence electrons to be shown in the Lewis structure of NO₃⁻ is
- 23
- 25
- 24
- 22
Hint and solution
Hint: Add the valence electrons of all atoms, then adjust for the charge on the ion. A negative charge means extra electrons.
- N: 5 e⁻; 3 O: 3 × 6 = 18 e⁻ — Nitrogen is in group 15 so it has 5 valence electrons. Each oxygen has 6, so three oxygens give 18.
- 5 + 18 = 23 — Adding the atoms gives 23 electrons for the neutral set of atoms.
- negative charge: +1 electron — A charge of −1 means the ion has gained one electron, so we add 1.
- 23 + 1 = 24 — The Lewis structure of NO₃⁻ must show 24 valence electrons. Option C.
Answer: C. N 5 + three O 18 + 1 for the negative charge = 24.
Q3. Statement I: Formal charges show the actual separation of charge in a molecule.
Statement II: Among possible Lewis structures, the one with the smallest formal charges has the lowest energy.
- Statement I is false but Statement II is true
- Both Statement I and Statement II are true
- Statement I is true but Statement II is false
- Both Statement I and Statement II are false
Hint and solution
Hint: Formal charge is an accounting method. Ask whether it describes real charge, and then recall the rule for choosing the best Lewis structure.
- FC = V − L − B/2 (bookkeeping) — Formal charge is calculated by assuming equal sharing of bonding electrons, so it is not an actual charge.
- Statement I: false — The real charge distribution depends on electronegativity, so formal charges do not show actual separation.
- Statement II: true — The structure with the smallest formal charges is the most stable one.
- I false, II true — So I is false and II is true. Option A.
Answer: A. Formal charges are only bookkeeping, so I is false. II is true.
How to use this chapter
Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.