Valency and Chemical Properties

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Valency: Combining capacity. Equal to electrons gained, lost or shared to attain octet.
Bond types: Ionic (metal + non-metal; electron transfer). Covalent (non-metal + non-metal; electron sharing). Metallic (sea of electrons).
Examples: NaCl ionic. H₂O, CO₂ covalent. Cu, Fe metallic.

Atoms combine to attain noble-gas configurations. CDS/OTA tests valency rules, three bond types and how bond type controls properties of compounds.

Valency

Valency = electrons in valence shell (if ≤ 4) or 8 minus electrons (if > 4).
Examples: Na (2,8,1) → 1, Mg (2,8,2) → 2, Al (2,8,3) → 3, C (2,4) → 4, N (2,5) → 3, O (2,6) → 2, F (2,7) → 1.
Some elements show variable valency — Fe (2 or 3 — ferrous/ferric), Cu (1 or 2 — cuprous/cupric), Sn (2 or 4).
Valencies determine chemical formulae — e.g. Na (1) + O (2) → Na₂O.

Ionic Bond

Complete transfer of electrons from one atom (usually metal) to another (usually non-metal).
Resulting cation and anion are held by electrostatic attraction.
Example — Na + Cl → Na⁺ + Cl⁻ → NaCl.
Properties — high melting point, brittle crystals, soluble in water, conduct electricity in molten or aqueous state.

Covalent Bond

Sharing of electrons between two non-metals.
Single bond (H₂, Cl₂), double bond (O₂, CO₂), triple bond (N₂).
Properties — low melting point, usually poor conductors, insoluble in water but soluble in organic solvents.
Polar covalent — sharing is unequal (H-Cl, H-O). Non-polar — equal sharing (H-H, Cl-Cl).

Metallic and Hydrogen Bonds

Metallic bond: a "sea of delocalised electrons" around a lattice of cations. Explains conductivity, malleability and lustre of metals.
Hydrogen bond: weak intermolecular attraction between H bonded to N, O or F and another electronegative atom. Explains anomalously high boiling point of water, DNA base pairing and ice being less dense than water.

CDS/OTA PYQ Examples

Q: NaCl is an example of a:

(a) Covalent compound (b) Ionic compound (c) Metallic compound (d) Coordination compound

Answer: (b) Ionic compound.

Q: Valency of carbon is:

(a) 2 (b) 3 (c) 4 (d) 6

Answer: (c) 4 — tetravalent.

Q: Which type of bond holds atoms in N₂?

(a) Single covalent (b) Double covalent (c) Triple covalent (d) Ionic

Answer: (c) Triple covalent — three shared pairs.

Q: High boiling point of water is due to:

(a) Covalent bonds (b) Hydrogen bonds (c) Ionic bonds (d) Metallic bonds

Answer: (b) Hydrogen bonding between water molecules.

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CDS/OTA-pattern items on Valency and Chemical Properties with answer keys and explanations.

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Frequently Asked Questions

Why do atoms form bonds?

To attain a stable noble-gas configuration (octet for most, duet for H), which is energetically favourable.

What is the difference between ionic and covalent compounds?

Ionic: electron transfer; high MP; soluble in water; conduct when molten or dissolved. Covalent: electron sharing; low MP; usually non-conductors; often insoluble in water.

What is a coordinate bond?

A covalent bond where both shared electrons come from one atom (donor). Example — NH₄⁺ (the H⁺ accepts a lone pair from N).