NDA Chemistry · Teaching notes

Chemical Bonding — NDA Chemistry

Chemical bonding is a small but concept-dense chapter in NDA Chemistry — about a dozen PYQs, mostly EASY and MODERATE, and almost all answerable from a few core ideas held firmly. The whole chapter rests on one driving force: every atom (except the noble gases) wants a full outer shell, so it bonds — by transferring electrons (ionic), by sharing them (covalent), or by one atom donating both shared electrons (coordinate). Metals pool their electrons in a sea (metallic). The chapter teaches in three movements, building from why atoms bond up to counting the bonds in a molecule: (1) Ionic and covalent bonding — the octet rule, the four bond types, ionic-vs-covalent character, bond polarity, and the lattice properties (melting point, conductivity) the bank tests as 'which is NOT correct' traps; (2) Valency, oxidation states and molecular formula — combining capacity, how to read valency from the group, oxidation states, and writing a formula by crossing valencies; (3) Bond counting and molecular structure — counting the covalent bonds in a small molecule, and odd-electron molecules that dimerize. Lewis-structure diagrams are kept for a later pass; this chapter teaches the bonding rules in words. Every PYQ is tagged — know the bond-type table and the valency rules cold, and the marks follow.

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Subtopic notes

PYQ weightage by concept

8 concepts · 11 PYQs — where the marks actually sit, so you know what to drill first

Ionic and Covalent Bonding5 PYQs · 45%

Concept	PYQs	Share
The octet rule and the four types of chemical bond	2	18%
Properties of ionic versus covalent compounds	2	18%
Bond polarity and polar molecules	1	9%

Valency, Oxidation States and Molecular Formula4 PYQs · 36%

Concept	PYQs	Share
Valency — combining capacity from the outer shell	2	18%
Oxidation state — the charge if every bond were ionic	1	9%
Writing a molecular formula by crossing valencies	1	9%

Bond Counting and Molecular Structure2 PYQs · 18%

Concept	PYQs	Share
Counting the covalent bonds in a molecule	1	9%
Odd-electron molecules and dimerization	1	9%

Formula & revision sheet

4 formulas · 3 reference tables · 11 gotchas across all subtopics — the exam-eve cheat-sheet

Ionic and Covalent Bonding

Reference tables (3)

The octet rule and the four types of chemical bond4 rows

Bond type	How the octet is reached	Formed between	Example
Ionic (electrovalent)	Electrons transferred (lost / gained)	Metal + non-metal	Na⁺Cl⁻, MgO
Covalent	Electrons shared (one pair from each atom)	Non-metal + non-metal	H₂O, CH₄, SiC
Coordinate (dative)	Shared pair donated by one atom only	Donor with a lone pair	NH₄⁺, H₃O⁺
Metallic	Valence electrons pooled in a 'sea'	Metal atoms	Na, Fe, Cu Metallic bonding (mobile electron sea) is why metals conduct electricity and are malleable.

Transfer = ionic; share = covalent; one-sided share = coordinate; pool = metallic.

Properties of ionic versus covalent compounds5 rows

Property	Ionic compounds	Covalent (molecular) compounds
Melting / boiling point	High (strong lattice)	Low (weak intermolecular forces) Among Na₂O, MgO, Fe₂O₃, CuO the highest melting point is MgO (~2852°C) — the small, doubly-charged Mg²⁺O²⁻ lattice.
Electrical conductivity	Conducts when molten or dissolved (not solid)	Does not conduct
Solubility	Usually soluble in water	Soluble in organic solvents
Physical state	Hard crystalline solids	Gases, liquids or soft solids
NaCl lattice coordination number	6 : 6 (each ion surrounded by 6 of the opposite)	—Q

Ionic = high MP + conducts when molten; covalent = low MP + does not conduct.

Bond polarity and polar molecules4 rows

Statement about water	True or false
Water is a polar molecule	TRUE — it is bent, so the O–H dipoles don't cancel The bank's trap: the FALSE option is 'water is a non-polar molecule'. Water is polar.
Water has a bent (V-shaped) geometry	TRUE — bond angle ≈ 104.5°
Water is a good solvent for ionic compounds	TRUE — its polarity pulls ions apart
Water is a non-polar molecule	FALSE — this is the statement the bank wants flagged

Watch out for (6)

Metal + non-metal is ionic; non-metal + non-metal is covalent
→ The octet rule and the four types of chemical bond
A coordinate bond is still a covalent bond
→ The octet rule and the four types of chemical bond
Ionic solids do NOT conduct — only when molten or dissolved
→ Properties of ionic versus covalent compounds
Higher charge → higher melting point
→ Properties of ionic versus covalent compounds
Water is polar, not non-polar
→ Bond polarity and polar molecules
Polar bonds don't always make a polar molecule
→ Bond polarity and polar molecules

Valency, Oxidation States and Molecular Formula

Formulas (3)

Valency — combining capacity from the outer shell · Common valencies $\text{Na}=1 \quad \text{Mg}=2 \quad \text{Al}=3 \quad \text{C}=4 \quad \text{N}=3 \quad \text{O}=2 \quad \text{Cl}=1$
Oxidation state — the charge if every bond were ionic · Sum rule for oxidation states $\sum (\text{oxidation states}) = \text{net charge}$
Writing a molecular formula by crossing valencies · Cross-over rule $A^{+m}\,B^{-n} \;\longrightarrow\; A_{n}B_{m}$

Watch out for (3)

Nitrogen's valency is 3, and nitride is N³⁻
→ Valency — combining capacity from the outer shell
Let the known atoms force the unknown
→ Oxidation state — the charge if every bond were ionic
Bracket a polyatomic ion before adding a subscript
→ Writing a molecular formula by crossing valencies

Bond Counting and Molecular Structure

Formulas (1)

Counting the covalent bonds in a molecule · Bonds formed per atom $\text{C}=4 \quad \text{N}=3 \quad \text{O}=2 \quad \text{H}=1 \quad \text{Cl}=1$

Watch out for (2)

Count every link, including the O–H in an alcohol
→ Counting the covalent bonds in a molecule
NO₂ dimerizes because it is a radical, not N₂O or N₂O₅
→ Odd-electron molecules and dimerization