Chapter in a nutshell: Atoms combine to attain a stable noble-gas (octet/duplet) configuration. They do this by transferring electrons (electrovalent/ionic bond), sharing electrons (covalent bond), or sharing a pair donated by one atom (coordinate/dative bond). The bond type decides whether a compound is a high-melting electrolyte (ionic) or a low-melting non-conductor (covalent).
1. Why Atoms Combine — the Octet Rule
Atoms react to achieve the stable electronic configuration of the nearest noble gas — a complete outer shell (octet = 8 electrons; duplet = 2 for H, Li, Be). They do so by losing, gaining or sharing valence electrons.2. Electrovalent (Ionic) Bond
Formed by the complete transfer of one or more electrons from a metal to a non-metal, producing oppositely charged ions held by electrostatic attraction.- Sodium chloride (NaCl): Na (2,8,1) → Na⁺ (2,8) + e⁻; Cl (2,8,7) + e⁻ → Cl⁻ (2,8,8). Na⁺ and Cl⁻ attract → NaCl.
- Magnesium chloride (MgCl₂): Mg → Mg²⁺ + 2e⁻; two Cl atoms each gain one electron → MgCl₂.
- Other examples: CaO, MgO, Na₂O, CaCl₂.
- Formed between elements of low ionisation energy (metals) and high electron affinity (non-metals).
Properties of electrovalent compounds:
| Property | Explanation |
|---|---|
| Hard crystalline solids | strong ionic lattice |
| High melting/boiling points | strong electrostatic forces |
| Conduct electricity when molten or in aqueous solution (not as solids) | ions become free to move |
| Soluble in water, insoluble in organic solvents | polar solvent separates ions |
| Undergo fast ionic reactions | ions already present |
3. Covalent Bond
Formed by the mutual sharing of electron pairs between two atoms (usually non-metals), each contributing one electron to a shared pair.| Type | Shared pairs | Examples |
|---|---|---|
| Single | 1 | H₂, Cl₂, HCl, H₂O, NH₃, CH₄ |
| Double | 2 | O₂, CO₂ |
| Triple | 3 | N₂ |
- H₂O: O shares with 2 H atoms (2 single bonds + 2 lone pairs).
- NH₃: N shares with 3 H atoms (1 lone pair). CH₄: C shares with 4 H atoms.
- CO₂: O=C=O (two double bonds). N₂: N≡N (triple bond).
Polar vs non-polar covalent:
- Non-polar: shared pair equally shared (identical atoms) — H₂, Cl₂, O₂.
- Polar: shared pair pulled towards the more electronegative atom — HCl, H₂O (partial charges).
Properties of covalent compounds:
| Property | Explanation |
|---|---|
| Liquids/gases or soft solids | weak inter-molecular forces |
| Low melting/boiling points | weak forces |
| Generally non-conductors | no free ions/electrons |
| Soluble in organic solvents | non-polar |
| Slow molecular reactions | bonds must break first |
4. Coordinate (Dative) Bond
A covalent bond in which the shared pair is contributed by only one atom (the donor); the other atom (acceptor) only provides an empty orbital. Shown by an arrow (→) from donor to acceptor.- Ammonium ion (NH₄⁺): NH₃ donates its lone pair to H⁺.
- Hydronium ion (H₃O⁺): H₂O donates a lone pair to H⁺.
- Once formed, a coordinate bond is identical to a normal covalent bond.
5. Ionic vs Covalent — Comparison
| Feature | Ionic (electrovalent) | Covalent |
|---|---|---|
| Formation | transfer of electrons | sharing of electrons |
| Particles | ions | molecules |
| State | solids | gases/liquids/soft solids |
| M.P / B.P | high | low |
| Conductivity | conduct molten/aqueous | non-conductors |
| Solubility | in water | in organic solvents |
| Reaction speed | fast (ionic) | slow (molecular) |
6. Worked / Structured Examples (ICSE pattern)
Q1. Show the formation of MgCl₂ by electron transfer. Solution: Mg (2,8,2) → Mg²⁺ (2,8) + 2e⁻; each Cl (2,8,7) gains 1e⁻ → 2Cl⁻ (2,8,8). Mg²⁺ + 2Cl⁻ → MgCl₂.Q2. Why does NaCl conduct electricity when molten but not when solid? Solution: In the solid the ions are fixed in the lattice; on melting (or dissolving) the ions become free to move and carry current.
Q3. Draw the bonding in a water molecule (describe). Solution: Oxygen (2,6) shares one electron with each of two hydrogen atoms → two O–H single (covalent) bonds; O retains two lone pairs. Each H attains a duplet, O an octet.
Q4. Identify the bond type: (i) CaO, (ii) CH₄, (iii) NH₄⁺. Solution: (i) ionic, (ii) covalent, (iii) covalent + one coordinate bond.
Q5. Why are covalent compounds generally poor conductors? Solution: They have no free ions or electrons to carry charge.
7. Key Terms — Quick Glossary
| Term | One-line definition |
|---|---|
| Octet rule | atoms react to attain 8 electrons in the outer shell. |
| Electrovalent bond | bond formed by transfer of electrons (ions). |
| Covalent bond | bond formed by sharing electron pairs. |
| Coordinate bond | covalent bond where both electrons come from one atom. |
| Lone pair | a valence electron pair not used in bonding. |
| Polar bond | covalent bond with unequal sharing (partial charges). |
| Electrovalency | number of electrons lost/gained. |
| Covalency | number of electrons shared. |
8. Common Mistakes to Avoid
- Saying ionic solids conduct in the solid state — they conduct only when molten or aqueous.
- Confusing electrovalency (transfer) with covalency (sharing).
- Forgetting that a coordinate bond's pair comes from one atom only.
- Calling H₂O or HCl "non-polar" — they are polar covalent.
- Drawing the wrong number of bonds (CO₂ has two double bonds; N₂ a triple bond).
9. Likely Exam Questions (with crisp answers)
- Why do atoms form chemical bonds? → To attain a stable noble-gas (octet/duplet) configuration.
- Define an electrovalent bond. → A bond formed by the complete transfer of electrons from one atom to another.
- Define a covalent bond. → A bond formed by the mutual sharing of electron pairs.
- What is a coordinate bond? Give an example. → A covalent bond where the shared pair comes from one atom; e.g. NH₄⁺.
- Why do ionic compounds have high melting points? → Strong electrostatic forces in the ionic lattice.
- Why do ionic compounds conduct electricity in molten/aqueous state? → The ions become free to move.
- Give two properties of covalent compounds. → Low melting points; generally non-conductors.
- Distinguish polar and non-polar covalent bonds. → Polar = unequal sharing (HCl); non-polar = equal sharing (H₂).
- How many electrons are shared in N₂? → Three pairs (a triple bond).
- Name the bond in NH₄⁺ formed between NH₃ and H⁺. → A coordinate (dative) bond.
- Why is electron transfer easy between a metal and a non-metal? → The metal has low ionisation energy; the non-metal has high electron affinity.
- State the type of bonding and particles in NaCl. → Ionic bonding; ions (Na⁺, Cl⁻).
- Why are covalent compounds usually volatile? → Weak inter-molecular forces → low boiling points.
- Define lone pair of electrons. → A valence electron pair not involved in bonding.
10. Electron-Dot (Lewis) Structures — step by step
- NaCl (ionic): Na• gives its 1 valence electron to Cl (which has 7 dots) → Na⁺ [no dots] and [Cl with 8 dots]⁻.
- MgO (ionic): Mg (2 valence e⁻) transfers both to O (6 dots) → Mg²⁺ and [O with 8 dots]²⁻.
- H₂ (covalent): H : H — one shared pair; each H now has a duplet.
- Cl₂ (covalent): each Cl brings 7 dots; they share one pair → both reach an octet, 3 lone pairs each.
- N₂ (triple): :N⦂⦂⦂N: — three shared pairs, one lone pair on each N.
- CO₂ (double): O::C::O — carbon shares two pairs with each oxygen (two double bonds).
- NH₄⁺ (coordinate): the N lone pair of NH₃ is donated to H⁺; the fourth N–H is a coordinate bond.
11. Exceptions & Special Cases
- Noble gases already have a full octet/duplet, so they do not normally bond.
- Giant covalent (network) solids — diamond, graphite, silica (SiO₂) — are covalent yet have very high melting points because the whole crystal is one network of covalent bonds. Graphite conducts electricity (free delocalised electrons) — an exception among covalent solids.
- Variable valency: some elements show more than one valency (Fe²⁺/Fe³⁺, Cu⁺/Cu²⁺).
12. More Worked Examples
Q6. Show the formation of CaO by electron transfer. Solution: Ca (2,8,8,2) → Ca²⁺ + 2e⁻; O (2,6) + 2e⁻ → O²⁻ (2,8). Ca²⁺ + O²⁻ → CaO. Q7. Why does CCl₄ not conduct electricity while NaCl solution does? Solution: CCl₄ is covalent (no ions); NaCl solution has free Na⁺ and Cl⁻ ions. Q8. State the number of shared and lone pairs in H₂O. Solution: 2 shared (bonding) pairs + 2 lone pairs on oxygen. Q9. Which has a higher boiling point: NaCl or HCl, and why? Solution: NaCl — it is ionic with a strong lattice; HCl is a covalent molecular gas with weak forces. Q10. Identify bond types in NH₃ and in MgCl₂. Solution: NH₃ — covalent (3 shared pairs); MgCl₂ — ionic.13. More Exam Questions (with crisp answers)
- Why is graphite a good conductor though it is covalent? → It has free (delocalised) electrons between its layers.
- Why does diamond have a very high melting point? → It is a giant covalent network; many strong covalent bonds must be broken.
- Give two differences between sodium chloride and carbon tetrachloride. → NaCl is ionic, high m.p., conducts when molten/aqueous; CCl₄ is covalent, low b.p., non-conductor.
- What is electrovalency? → The number of electrons an atom loses or gains to form ions.
- What is covalency? → The number of electron pairs an atom shares.
- Why do ionic reactions occur faster than covalent reactions? → Ions are already present, so no bonds need to be broken first.
- Name a molecule with a triple bond and one with a double bond. → N₂ (triple); O₂ or CO₂ (double).
- Why is water a polar molecule? → Oxygen is more electronegative, pulling the shared electrons, giving partial charges.
14. How to Predict the Bond Type (quick rule)
- Metal + non-metal → usually ionic (e.g. NaCl, MgO, CaCl₂).
- Non-metal + non-metal → usually covalent (e.g. CO₂, NH₃, H₂O).
- A large electronegativity difference → ionic; a small difference → covalent (polar if not zero).
15. Shapes (for reference)
- H₂O — bent/angular (two lone pairs on O).
- NH₃ — pyramidal (one lone pair on N).
- CH₄ — tetrahedral (no lone pairs).
- CO₂ — linear.
16. Final Quick-Revision Q&A
- Which bond is formed when electronegativity difference is very large? → Ionic (electrovalent).
- State one test to distinguish an ionic from a covalent compound. → Check electrical conductivity in molten/aqueous state — ionic conducts, covalent does not.
- Why do covalent compounds dissolve in organic solvents? → They are non-polar/weakly polar and mix with non-polar solvents.
- Name the ion formed when water accepts a proton. → Hydronium ion (H₃O⁺), via a coordinate bond.
- How many lone pairs does nitrogen have in NH₃? → One.