Valency and Chemical Bonding

Atoms combine to achieve stable electron configurations (typically octet). NDA tests three bond types, valency, and properties of compounds.

Valency

• Valency = electrons in outermost shell (if ≤4) or 8 minus that (if >4).
• Indicates how many bonds an atom typically forms.
• Examples:
  • Na (2,8,1) — valency 1 (loses 1 electron).
  • Mg (2,8,2) — valency 2.
  • Al (2,8,3) — valency 3.
  • C (2,4) — valency 4.
  • N (2,5) — valency 3 (gains 3 to reach octet).
  • O (2,6) — valency 2.
  • F (2,7) — valency 1.
• Some elements have variable valency: Fe (2 or 3), Cu (1 or 2), N (3 or 5).

Ionic Bond

• Formation by complete transfer of electrons from one atom (usually metal) to another (usually non-metal).
• Resulting ions: cation (+) and anion (-) held together by electrostatic attraction.
• NaCl example: Na (2,8,1) loses 1 e⁻ → Na⁺ (2,8). Cl (2,8,7) gains 1 e⁻ → Cl⁻ (2,8,8). Na⁺ + Cl⁻ → NaCl.
• Properties of ionic compounds:
  • Solid at room temperature, crystalline structure.
  • High melting and boiling points.
  • Conduct electricity when molten or dissolved (free ions). Not in solid state.
  • Soluble in water, insoluble in organic solvents.

Covalent Bond

• Formation by sharing of electrons between atoms (usually non-metals).
• Single bond: 1 pair shared (H-H, Cl-Cl).
• Double bond: 2 pairs shared (O=O, CO₂).
• Triple bond: 3 pairs shared (N≡N).
• Polar covalent: Electrons not shared equally; one end is partially +ve and other -ve. e.g., HCl, H₂O.
• Non-polar covalent: Equal sharing. e.g., H₂, Cl₂, O₂.
• Properties of covalent compounds:
  • Often gas or liquid; some solids (sugars, polymers).
  • Lower melting/boiling points than ionic.
  • Generally don't conduct electricity (no free ions/electrons).
  • Often soluble in organic solvents, less in water.

Metallic and Hydrogen Bonds

Metallic bond:

• Found in pure metals and alloys.
• Outer electrons form a "sea" delocalised over a lattice of positive metal ions.
• Properties: Good electrical and thermal conductivity, ductility, malleability, lustre.

Hydrogen bond:

• Weak attraction between H atom bonded to highly electronegative atom (N, O, F) and another electronegative atom.
• Explains: high boiling point of water, ice floating on water (less dense), DNA base pairing, protein structure.

Octet Rule and Stability

• Octet Rule: Atoms tend to have 8 electrons in their valence shell (or 2 for first shell — duplet rule).
• Exceptions: H, He, Li (2 valence electrons), and some heavier atoms (B, P, S) that can have less or more than 8.
• Noble gases (He, Ne, Ar, Kr, Xe, Rn) already have 2 or 8 outer electrons → naturally stable, hence "inert".

NDA PYQ Examples

Frequently Asked Questions

Why is water a liquid at room temperature while H₂S is a gas?

Hydrogen bonding. Water (H-O-H) forms strong intermolecular hydrogen bonds, raising its boiling point to 100°C. H₂S can't form H-bonds (S is less electronegative than O), so it remains a gas (boiling point -60°C).

Why are ionic compounds soluble in water?

Water is a polar solvent. Its partial charges (δ+ on H, δ- on O) attract and surround the cations and anions, separating them and dissolving the ionic lattice. The energy released in hydration exceeds the lattice energy.

Why are diamonds so hard?

Each carbon is covalently bonded to four others in a 3D tetrahedral lattice. To break a diamond, you must break millions of strong covalent bonds. Hardest natural material — 10 on Mohs scale.

Why do metals conduct electricity?

Metallic bonding creates a 'sea' of mobile electrons that can flow when a potential difference is applied. Copper and silver have especially mobile electrons → best conductors.

What is the difference between molecular and ionic compounds?

Molecular: discrete molecules with covalent bonds (sugar, water, methane). Lower melting points, often non-conducting. Ionic: lattice of cations and anions (salt, baking soda). Higher melting points, conduct when molten/dissolved.

Related Topics

NDA ChemistryAtomic Structure NDA ChemistryIsotopes NDA ChemistryAcids & Bases NDA ChemistryMetals & Alloys NDAChemistry — Full Syllabus