Chemical Bonding ICSE Class 10: Electrovalent, Covalent and Coordinate Bonds

What Chemical Bonding is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Chemistry. Chemical Bonding is in the 2. Chemical Bonding part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2027

Key concepts

Why atoms bond: duplet and octet

Noble gases rarely react because their outer shells are full (helium 2; neon 2, 8; argon 2, 8, 8). Other atoms gain, lose or share electrons to reach the same arrangement.

• A metal with 1, 2 or 3 outer electrons loses them to form a cation (Na⁺, Mg²⁺, Ca²⁺).
• A non-metal with 5, 6 or 7 outer electrons gains to form an anion (Cl⁻, O²⁻) when it meets a metal, or shares electrons when it meets another non-metal.

The charge on an ion equals the number of electrons lost (positive) or gained (negative).

Worked example

Write the electronic configurations of Mg, Mg²⁺, O and O²⁻ (atomic numbers 12 and 8).

Try it first, then show the solutionSolution

1. Mg: 2, 8, 2; losing 2 electrons gives Mg²⁺: 2, 8.
2. O: 2, 6; gaining 2 electrons gives O²⁻: 2, 8.
3. Both ions now have the neon arrangement.

Answer: Mg 2, 8, 2; Mg²⁺ 2, 8; O 2, 6; O²⁻ 2, 8.

Electrovalent bonding: NaCl, MgCl₂, CaO

An electrovalent bond is the electrostatic attraction between oppositely charged ions formed by transfer of electrons from a metal to a non-metal.

• NaCl: Na (2, 8, 1) gives 1 electron to Cl (2, 8, 7). Ions: Na⁺ (2, 8) and Cl⁻ (2, 8, 8).
• MgCl₂: Mg (2, 8, 2) gives 1 electron to each of two Cl atoms. Ions: Mg²⁺ and 2Cl⁻.
• CaO: Ca (2, 8, 8, 2) gives 2 electrons to O (2, 6). Ions: Ca²⁺ (2, 8, 8) and O²⁻ (2, 8).

In a dot structure, show the metal's electrons as crosses and the non-metal's as dots, draw an arrow for each transferred electron, then write each ion in square brackets with its charge outside.

Worked example

Explain the formation of magnesium chloride by electron transfer. Write the formula and state the charges.

Try it first, then show the solutionSolution

1. Mg has 2 outer electrons and needs to lose both.
2. Each Cl atom has 7 and needs 1 more, so two Cl atoms are needed.
3. Mg gives one electron to each Cl: Mg²⁺ and two Cl⁻ ions.
4. Total positive charge 2+ balances total negative charge 2 × 1- = 2-.

Answer: MgCl₂: one Mg²⁺ ion and two Cl⁻ ions.

Properties of electrovalent compounds

• State: hard crystalline solids at room temperature, because strong electrostatic forces hold the ions in a lattice.
• Melting and boiling points: high, as much energy is needed to break the lattice.
• Conductivity: do not conduct electricity as solids (ions are fixed), but do conduct when molten or in aqueous solution, because the ions are then free to move. This is why they are electrolytes.
• Solubility: usually soluble in water, insoluble in organic solvents such as kerosene or benzene.
• Dissociation: ions already exist in the solid; water or heat only frees them: $NaCl\to N{a}^{+}+C{l}^{-}$.

Worked example

Solid sodium chloride does not conduct electricity, but molten sodium chloride does. Explain.

Try it first, then show the solutionSolution

1. Solid NaCl consists of Na⁺ and Cl⁻ ions locked in a crystal lattice.
2. On melting, the lattice breaks down and the ions become mobile.
3. Moving ions carry the current.

Answer: Ions are fixed in the solid but free to move in the melt.

Non-polar covalent bonding

A covalent bond is formed by mutual sharing of one or more electron pairs between two non-metal atoms. One shared pair is a single bond, two a double bond, three a triple bond.

• H₂: H•×H, one shared pair; each H gets a duplet.
• Cl₂: one shared pair; each Cl has 3 lone pairs.
• O₂: two shared pairs (double bond); each O has 2 lone pairs.
• N₂: three shared pairs (triple bond); each N has 1 lone pair.
• CH₄: carbon shares one pair with each of four H atoms; no lone pairs on C.
• CCl₄: four C–Cl single bonds; each Cl keeps 3 lone pairs.

When identical atoms share, the pair is shared equally: non-polar. CH₄ and CCl₄ are non-polar molecules because their four bonds are arranged symmetrically (tetrahedrally), so any pulls cancel.

Worked example

How many shared pairs and lone pairs are there in a nitrogen molecule?

Try it first, then show the solutionSolution

1. N has 5 outer electrons and needs 3 more.
2. Each N shares 3 electrons: 3 shared pairs (a triple bond).
3. Each N keeps 2 unshared electrons: 1 lone pair each.

Answer: 3 shared pairs and 2 lone pairs (one on each N).

Polar covalent bonding: HCl, H₂O, NH₃

When two different atoms share electrons, the more electronegative atom pulls the shared pair towards itself. It gets a partial negative charge (δ−) and the other atom a partial positive charge (δ+). The bond is polar covalent.

• HCl: H shares 1 pair with Cl; Cl (3 lone pairs) is δ−, H is δ+.
• H₂O: O shares one pair with each H; O keeps 2 lone pairs; O is δ−, each H δ+. The molecule is bent, so it is polar.
• NH₃: N shares one pair with each of three H atoms; N keeps 1 lone pair; N is δ−.

Polar covalent compounds such as HCl can ionise in water even though they contain no ions as pure substances.

Worked example

Why is HCl described as a polar covalent molecule and not an ionic compound?

Try it first, then show the solutionSolution

1. H and Cl are both non-metals, so they share a pair rather than transfer an electron.
2. Cl is more electronegative, so the shared pair is closer to Cl.
3. This gives partial charges (δ+ on H, δ− on Cl), not full ionic charges.

Answer: Electrons are shared unequally: polar covalent.

Properties of covalent compounds and the comparison

Covalent compounds exist as separate molecules held together by weak forces (van der Waals forces) between molecules, so:

• State: gases, liquids or soft solids (CH₄ and HCl gases, H₂O and CCl₄ liquids).
• Melting and boiling points: low, since only weak intermolecular forces must be overcome.
• Conductivity: poor conductors of heat and electricity; they contain no free ions. Polar ones such as HCl ionise in water and then conduct: $HCl+H_{2}O\to H_3{O}^{+}+C{l}^{-}$.
• Solubility: generally soluble in organic solvents, mostly insoluble in water (polar ones like HCl and NH₃ are exceptions).

Property	Electrovalent	Covalent
Formed by	transfer of electrons	sharing of electrons
Particles	ions	molecules
State	crystalline solids	gases, liquids, soft solids
m.p./b.p.	high	low
Conduction	when molten or in water	non-conductors (polar ones ionise in water)
Solvent	water	organic solvents

Worked example

Carbon tetrachloride is a liquid that does not conduct electricity. Give two reasons from its bonding.

Try it first, then show the solutionSolution

1. It is made of separate CCl₄ molecules with weak forces between them, so its boiling point is low: it is a liquid.
2. Its bonds are covalent and it has no ions or free electrons, so it cannot carry current.

Answer: Weak intermolecular forces (liquid); no free ions (non-conductor).

Coordinate bonding: hydronium and ammonium ions

A lone pair is a pair of outer electrons not used in bonding. A coordinate bond is a covalent bond in which both shared electrons come from one atom (the donor); the other atom (the acceptor) needs a pair, like a hydrogen ion H⁺ which has no electrons.

• Hydronium ion: the oxygen of water donates one of its two lone pairs to H⁺: $H_{2}O+H^{+}\to H_3{O}^{+}$. H₃O⁺ has three O–H bonds and one lone pair left on O.
• Ammonium ion: the nitrogen of ammonia donates its only lone pair to H⁺: $N{H}_3+H^{+}\to N{H}_4^{+}$. NH₄⁺ has four N–H bonds and no lone pair.

Once formed, the coordinate bond is just like the other bonds; the + charge belongs to the whole ion, so write it outside square brackets. In a dot diagram, the coordinate pair can be shown with an arrow from the donor to H⁺ before bonding.

Worked example

How many lone pairs are there on the central atom in (a) H₂O, (b) H₃O⁺, (c) NH₃, (d) NH₄⁺?

Try it first, then show the solutionSolution

1. H₂O: O has 6 outer electrons, uses 2 in bonding, keeps 2 lone pairs.
2. H₃O⁺: one lone pair is donated to H⁺, so 1 remains.
3. NH₃: N has 5, uses 3, keeps 1 lone pair.
4. NH₄⁺: that lone pair is donated, so none remains.

Answer: (a) 2 (b) 1 (c) 1 (d) 0

Chemistry in real life: Chemical Bonding

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Covalent Bonding: Polar and Non-polar MoleculesComing soon
• Electrovalent (Ionic) Bonding
• Coordinate Bonding: Hydronium and Ammonium IonsComing soon
• Coordinate Bonding: Hydronium and Ammonium Ions
• Covalent Bonding, Polar and Non-polar Molecules

Chemical Bonding formula sheet

Formation of the hydronium ion

$H_{2}O+H^{+}\to H_3{O}^{+}$

Coordinate bond from a lone pair on oxygen.

Formation of the ammonium ion

$N{H}_3+H^{+}\to N{H}_4^{+}$

Coordinate bond from the lone pair on nitrogen.

Ionisation of HCl in water

$HCl+H_{2}O\to H_3{O}^{+}+C{l}^{-}$

A polar covalent compound gives ions in water.

Dissociation of an ionic compound

$MgC{l}_2\to M{g}^{2+}+2C{l}^{-}$

Ions already exist; melting or dissolving frees them.

Electron transfer in CaO

$Ca\to C{a}^{2+}+2e^{-};O+2e^{-}\to O^{2-}$

Charges equal electrons lost or gained.

Chemical Bonding: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Chemical Bonding

The question patterns to expect, and the method that works for each.

Tricks to remember Chemical Bonding

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Lone pairs: 3-2-1-0

  Count across from chlorine: Cl in HCl has 3 lone pairs, O in H₂O has 2, N in NH₃ has 1, C in CH₄ has 0. Each step left loses one lone pair and gains one bond.

  Use it when: Lone pair MCQs and dot diagrams.

• Give one, lose one

  Every coordinate bond to H⁺ uses up one lone pair: H₂O (2) becomes H₃O⁺ (1); NH₃ (1) becomes NH₄⁺ (0).

  Use it when: Lone pairs in the ions.

• Metal + non-metal = ions

  Metal meets non-metal: transfer (ionic). Non-metal meets non-metal: share (covalent).

  Use it when: Bond type from a table of letters.

• Ions are hard and hot, molecules are soft and cool

  Ionic: hard solids, high melting point. Covalent: gases, liquids or soft solids, low melting point.

  Use it when: Comparison tables.

Chemical Bonding: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 6-mark question split into 2 + 2 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Chemical Bonding

The slips examiners see most often, and what to write instead.

• ✗Mistake: Drawing bond lines with the dots

  CISCE examiners' comments, 2024: in electron dot structures, candidates drew lines between dots or between a dot and a cross, used the same symbol for both atoms, and omitted the lone pair on each nitrogen.

  Wrong working

  N≡N drawn with dots and three lines, no lone pairs.

  Right working

  Three dot-cross pairs between the N atoms and one lone pair on each N; no lines.

  ✓Do this: Dots for one atom, crosses for the other, no lines, and count every outer electron.

  Usually costs: 1-2 marks.

• ✗Mistake: Leaving out the charges in an ionic structure

  CISCE examiners' comments, 2025: in the dot and cross structure of MgCl₂, candidates missed the ionic charges, showed covalent sharing, or did not distinguish Mg and Cl electrons.

  Wrong working

  Mg shares a pair with each Cl.

  Right working

  [Mg]²⁺ 2[Cl]⁻, with the two transferred electrons shown as crosses on the chlorides.

  ✓Do this: Ionic structures always show transfer arrows, brackets and charges.

  Usually costs: 1-2 marks.

• ✗Mistake: Wrong lone pairs in water and ammonia

  CISCE examiners' comments, 2024: candidates gave water's oxygen one lone pair (confusing it with ammonia) and sometimes gave hydrogen a lone pair.

  Wrong working

  Oxygen in water has one lone pair.

  Right working

  Oxygen in water has two lone pairs; nitrogen in ammonia has one; hydrogen has none.

  ✓Do this: Use the 3-2-1-0 rule from Cl to C.

  Usually costs: 1 mark.

• ✗Mistake: Vague reasons for inertness and low melting points

  CISCE examiners' comments, 2024: candidates wrote 'they are inert' instead of 'stable electronic configuration', and 'less force' without naming the weak forces between molecules.

  Wrong working

  Noble gases do not react because they are inert.

  Right working

  Noble gases have a complete (stable) outer shell, so they have no tendency to lose, gain or share electrons.

  ✓Do this: Every reason names the cause: configuration, particles, or forces between them.

  Usually costs: 1 mark per reason.

• ✗Mistake: Identifying letter elements and wrong formulae

  CISCE examiners' comments, 2024: when atoms were given as letters, candidates named the elements (CaO, CCl₄) instead of using letters, or wrote A₂B₂.

  Wrong working

  A (2, 8, 8, 2) with B (2, 6): CaO

  Right working

  AB (both valency 2, simplified ratio)

  ✓Do this: Use only the letters; criss-cross and simplify.

  Usually costs: 1 mark.

• ✗Mistake: Calling CCl₄ polar

  CISCE examiners' comments, 2024: in a fill-in-the-blank, candidates chose 'polar' for carbon tetrachloride.

  Wrong working

  CCl₄ is a polar covalent compound.

  Right working

  CCl₄ is non-polar: its symmetrical shape cancels the pulls of the polar bonds.

  ✓Do this: The syllabus lists CCl₄ with the non-polar examples.

  Usually costs: 1 mark.

Practice questions on Chemical Bonding

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which molecule contains a triple bond?
   (a) O₂
   (b) N₂
   (c) Cl₂
   (d) CH₄

   Hint

   Valency 3.

   Answer

   (b) N₂

2. Q2 · 1 mark · MCQBoard level

   The compound with the highest melting point is:
   (a) CCl₄
   (b) NH₃
   (c) CaO
   (d) HCl

   Hint

   Ionic lattice.

   Answer

   (c) CaO

3. Q3 · 1 mark · MCQHarder board level

   An atom X (2, 8, 6) combines with hydrogen. The number of lone pairs on X in the compound is:
   (a) 0
   (b) 1
   (c) 2
   (d) 3

   Hint

   Like oxygen in water.

   Answer

   (c) 2: X forms XH₂ and keeps two lone pairs.

4. Q4 · 1 mark · Assertion-reasonBoard level

   Assertion (A): The ammonium ion has no lone pair of electrons.
   Reason (R): The lone pair of nitrogen is used to form a coordinate bond with H⁺.
   (a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
   (c) (A) is true and (R) is false.
   (d) (A) is false and (R) is true.

   Hint

   Where does the pair go?

   Answer

   (a)

5. Q5 · 2 marks · Short answerBoard level

   Draw the electron dot structures of (a) chlorine molecule, (b) methane.

   Hint

   Count lone pairs.

   Answer

   (a) Cl–Cl: one shared pair, three lone pairs on each Cl. (b) C in the centre sharing one pair with each of four H atoms; no lone pairs.

6. Q6 · 3 marks · Short answerHarder board level

   Element A (atomic number 19) combines with element B (atomic number 17). (a) State the type of bond. (b) Write the formula. (c) Would the compound conduct electricity in the solid state? Explain.

   Hint

   A metal and a halogen.

   Answer

   (a) Electrovalent (b) AB (c) No: ions are fixed in the solid lattice.

7. Q7 · 3 marks · Short answerHarder board level

   Give reasons: (a) Ammonia gas readily dissolves in water. (b) Electrovalent compounds are hard solids. (c) Methane does not conduct electricity.

   Hint

   Polarity; lattice; no ions.

   Answer

   (a) Ammonia is a polar covalent molecule, like water. (b) Strong electrostatic forces hold the ions in a lattice. (c) It is covalent with no free ions or electrons.

8. Q8 · 4 marks · Long answerHarder board level

   Explain, with electron dot diagrams, the formation of the hydronium ion and the ammonium ion. State one difference in the number of lone pairs.

   Hint

   Each uses one lone pair.

   Answer

   H₂O donates one of its two lone pairs to H⁺: $H_{2}O+H^{+}\to H_3{O}^{+}$ (one lone pair left). NH₃ donates its only lone pair: $N{H}_3+H^{+}\to N{H}_4^{+}$ (no lone pair left). Diagrams as in the draw-and-label checklist.

9. Q9 · 3 marks · Short answerHOTS / competency

   Element M forms the ion M²⁺ with the configuration 2, 8. Element N forms N³⁻ with the configuration 2, 8. (a) Find the atomic numbers. (b) Write the formula of the compound of M and N. (c) Is it likely to be soluble in kerosene?

   Hint

   Add back lost electrons; remove gained ones.

   Answer

   (a) M = 12, N = 7. (b) Criss-cross 2 and 3: M₃N₂. (c) No: it is ionic, and ionic compounds are insoluble in organic solvents.

10. Q10 · 3 marks · Short answerHarder board level

    A molecule YZ has a shared pair closer to Z. Another molecule Z₂ has the pair shared equally. (a) Name the bond type in each. (b) Which of Y and Z is more electronegative? (c) Give one example of each from the syllabus.

    Hint

    Unequal and equal sharing.

    Answer

    (a) YZ polar covalent; Z₂ non-polar covalent. (b) Z. (c) HCl and Cl₂.

11. Q11 · 2 marks · Short answerHOTS / competency

    A student draws the dot structure of oxygen with one shared pair and three lone pairs on each atom. Find the error, correct it, and state the bond type.

    Hint

    Count to eight.

    Answer

    That drawing gives each O atom 7 electrons of its own (1 in the shared pair + 6 in lone pairs), but oxygen has only 6 outer electrons. Oxygen needs 2 more electrons, so each atom shares two of its electrons: two shared pairs (a double bond) and two lone pairs on each O. Bond type: non-polar covalent double bond.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Define an electrovalent bond.Show answer

   The bond formed by transfer of electrons from one atom to another, giving oppositely charged ions held by electrostatic attraction.

2. 2.Define a covalent bond.Show answer

   A bond formed by mutual sharing of one or more electron pairs between two atoms.

3. 3.Define a coordinate bond.Show answer

   A covalent bond in which both shared electrons are provided by one atom.

4. 4.What is a lone pair?Show answer

   A pair of outer-shell electrons not used in bonding.

5. 5.Lone pairs on O in water?Show answer

   Two.

6. 6.Type of bond in N₂?Show answer

   Triple covalent bond.

7. 7.Which ion is formed when water accepts H⁺?Show answer

   Hydronium ion, H₃O⁺.

8. 8.Why does molten NaCl conduct?Show answer

   Its ions are free to move.

9. 9.Name a polar covalent molecule from the syllabus.Show answer

   HCl, H₂O or NH₃.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you draw NaCl, MgCl₂ and CaO with arrows, brackets and charges?
• ☐Can you draw N₂ and O₂ with the right number of shared and lone pairs?
• ☐Do you know which syllabus molecules are polar and which are non-polar?
• ☐Can you explain why CCl₄ is non-polar?
• ☐Can you draw the formation of NH₄⁺ and H₃O⁺?
• ☐Can you write four differences between electrovalent and covalent compounds?

Chemical Bonding: frequently asked questions