NDA Chemistry · Chemical Bonding
Ionic and Covalent Bonding
Atoms bond to complete their outer shells — metals give electrons to non-metals (ionic), non-metals share electrons (covalent), one atom can donate both shared electrons (coordinate), and metals pool their electrons (metallic).
Why this matters
The foundation of the whole chapter — 5 PYQs and the frame for every other concept. The bank tests it four ways: spot the covalent compound in a list of ionic ones, rank oxides by melting point (ionic strength), give the coordination number in an ionic lattice, and the classic 'which statement about water is NOT correct' polarity trap. Learn the octet rule, the four bond types, and the ionic-vs-covalent property contrast, and these all fall out.
Concept 1 of 3
The octet rule and the four types of chemical bond
Intuition
Noble gases (He, Ne, Ar…) are unreactive because their outer shell is already full. Every other atom bonds to reach that same full outer shell of eight electrons — the octet. There are only four ways to do it: hand electrons over (ionic), share them (covalent), have one atom donate both shared electrons (coordinate), or pool them across many atoms (metallic).
Definition
Atoms bond to achieve a stable, completely filled outermost shell (a duplet of 2 for hydrogen and lithium, an octet of 8 for most others) — the octet rule. The four bond types:
- Ionic (electrovalent) bond — a metal transfers electrons to a non-metal, forming positive and negative ions held by electrostatic attraction. Example: Na⁺Cl⁻.
- Covalent bond — two non-metals share one or more pairs of electrons. Example: H₂O, CH₄, SiC.
- Coordinate (dative) bond — a covalent bond where both shared electrons come from the same atom. Example: the fourth N–H bond in the ammonium ion NH₄⁺.
- Metallic bond — metal atoms release their valence electrons into a shared 'sea' of delocalised electrons around fixed positive ions.
| 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.
Practice this conceptself-check · 6 quick reps
Try it yourself
Which one of these is a covalent compound: calcium oxide, sodium nitride, silicon carbide, or zinc sulphide?
Practice — Level 1 (6 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Why do atoms form chemical bonds?
- 2.What type of bond forms when a metal transfers electrons to a non-metal?
- 3.What type of bond forms when two non-metals share electrons?
- 4.In which bond do both shared electrons come from the same atom?
- 5.Which bond type holds a piece of copper metal together?
- 6.Is silicon carbide (SiC) ionic or covalent?
From the bank · past-year question
Which one of the following is a covalent compound?
[Q61 · Apr · 2022]
Metal + non-metal is ionic; non-metal + non-metal is covalent
The quick test: a compound of a metal and a non-metal (CaO, Na₃N, ZnS) is ionic; a compound of two non-metals (SiC, CO₂, H₂O) is covalent. Silicon carbide trips students because it 'looks hard like a salt', but Si and C are both non-metals → covalent.
A coordinate bond is still a covalent bond
A coordinate (dative) bond shares a pair of electrons just like an ordinary covalent bond — the only difference is that one atom supplied both electrons. Once formed it is identical to any other covalent bond.
Concept 2 of 3
Properties of ionic versus covalent compounds
Intuition
Ionic compounds are giant lattices of ions held by strong electrostatic forces — so they melt high and conduct when molten or dissolved. Covalent compounds are usually small molecules held to each other weakly — so they melt low and don't conduct. Stronger ionic attraction (higher charges, smaller ions) means a higher melting point: that is how the bank ranks oxides.
Definition
The property contrast the bank tests, and why:
- Melting / boiling point — ionic: high (strong lattice forces); covalent (molecular): low (weak forces between molecules). Higher ion charges raise the melting point, so MgO (Mg²⁺O²⁻) melts higher than Na₂O (Na⁺O²⁻) — a +2/−2 lattice beats a +1/−2 one.
- Electrical conductivity — ionic: conduct when molten or in solution (ions are then free to move), but not as a solid; covalent: do not conduct (no free ions or electrons).
- Solubility — ionic compounds usually dissolve in water (polar); covalent (non-polar) compounds dissolve in organic solvents.
- State — ionic compounds are hard crystalline solids; covalent compounds are often gases, liquids or soft solids.
- Coordination number — in the NaCl lattice each Na⁺ is surrounded by 6 Cl⁻ and each Cl⁻ by 6 Na⁺, so the coordination number is 6 : 6.
| 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.
Practice this conceptself-check · 5 quick reps
Try it yourself
Which oxide has the highest melting point: Na₂O, MgO, Fe₂O₃ or CuO?
Practice — Level 1 (5 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Do ionic or covalent compounds generally have higher melting points?
- 2.When does an ionic compound conduct electricity?
- 3.Do covalent (molecular) compounds conduct electricity?
- 4.What is the coordination number of Na⁺ and Cl⁻ in the NaCl lattice?
- 5.Why does MgO melt higher than Na₂O?
From the bank · past-year question
Which one among the following oxides has the highest melting point ?
[Q107 · Apr · 2024]
Ionic solids do NOT conduct — only when molten or dissolved
A statement that 'solid sodium chloride conducts electricity' is wrong. In the solid the ions are locked in the lattice; they conduct only when molten or dissolved, when the ions can move.
Higher charge → higher melting point
When ranking oxide melting points, the +2/−2 lattice (MgO) beats the +1/−2 lattice (Na₂O). Don't rank by molecular mass — rank by ionic charge and size.
Concept 3 of 3
Bond polarity and polar molecules
Intuition
When two different atoms share electrons, the more electronegative atom pulls the shared pair closer, so one end is slightly negative and the other slightly positive — a polar bond. If a molecule's bent or asymmetric shape leaves those pulls uncancelled, the whole molecule is polar. Water is the classic polar molecule the bank loves to falsify.
Definition
Polarity facts the bank tests:
- A bond between two identical atoms (H–H, Cl–Cl) is non-polar — the electrons are shared equally.
- A bond between different atoms (H–O, H–Cl) is polar — the more electronegative atom (O, Cl) gains a partial negative charge δ⁻ and the other a partial positive δ⁺.
- Water (H₂O) is a polar molecule — it is bent (~104.5°), so the two O–H bond dipoles don't cancel; oxygen is δ⁻, the hydrogens δ⁺. This polarity makes water an excellent solvent for ionic compounds and gives it a high boiling point (hydrogen bonding).
- A symmetric molecule (CO₂, CH₄) can have polar bonds yet be non-polar overall because the bond dipoles cancel.
| 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 |
Practice this conceptself-check · 5 quick reps
Try it yourself
Which statement about water is NOT true: (a) it is a polar molecule, (b) it has a bent shape, (c) it is a non-polar molecule, (d) it dissolves ionic salts?
Practice — Level 1 (5 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Is the H–H bond polar or non-polar?
- 2.Is the H–O bond polar or non-polar?
- 3.Is water a polar or non-polar molecule?
- 4.Why is water a good solvent for salts?
- 5.Why is CO₂ non-polar despite having polar C=O bonds?
From the bank · past-year question
Which one of the following statements about water is true?
[Q61 · Sep · 2019]
Water is polar, not non-polar
The single most-tested trap in this subtopic: 'water is a non-polar molecule' is FALSE. Water is polar because its bent shape stops the two O–H bond dipoles cancelling.
Polar bonds don't always make a polar molecule
CO₂ has polar C=O bonds but is non-polar overall because the molecule is linear and the dipoles cancel. Shape decides molecular polarity, not just the bonds.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
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
Mastery check — 2 interleaved questions
Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.
How many covalent bonds are present in a Chloropropane molecule having molecular formula ?
[Q93 · Apr · 2020]
What is the coordination number of and ions in NaCl lattice ?
[Q102 · Apr · 2024]
Drill every past-year question on this subtopic
5 questions from the bank — paginated, with cart and Word-export support.