Mini Chemistry
Part of Mini EducationSwitch subject

Covalent Bonds

Covalent bonding: definition (shared electrons), dot-and-cross diagrams, single/double/triple bonds, and simple vs giant covalent properties.

  • SEC G3 Pure Chemistry 2027
On this page

Learning objectives

  • describe the formation of a covalent bond by the sharing of a pair of electrons and that the atoms in the molecules usually have the electronic configuration of a noble gas
  • describe, using ‘dot-and-cross’ diagrams, the formation of covalent bonds between non-metallic elements, e.g. H2; O2; H2O; CH4; CO2
  • deduce the arrangement of electrons in other covalent molecules
  • relate the physical properties (including electrical property) of covalent substances to their structure and bonding (see also 3.4(g)).

Covalent bonding questions become easier when you picture two atoms sharing electrons. Start with the syllabus idea—a shared pair of electrons—then use the attraction between that pair and both nuclei when a question asks you to explain the bond.

1. Definition

A covalent bond is formed when two atoms share a pair of electrons. The shared pair is attracted to both positively charged nuclei, holding the atoms together.

It usually occurs between non-metal atoms.

2. Key Ideas

  • Covalent bonding involves electron sharing (not transfer).
  • A single bond = 1 shared pair of electrons.
  • A double bond = 2 shared pairs of electrons.
  • A triple bond = 3 shared pairs of electrons.
  • Most covalent substances are simple molecular (low melting/boiling points) because forces between molecules are weak.
  • Some covalent substances form giant covalent structures (very high melting points) because many strong covalent bonds must be broken.

3. Detailed Explanations

Quick Recall (covalent keywords)
  • Covalent bonding = sharing electrons (not transfer).
  • A shared pair is attracted to both nuclei, holding the atoms together.
  • Simple molecular substances have low melting/boiling points due to weak intermolecular forces.

A. How Covalent Bonds Form (Electron Sharing)

Non-metals bond by sharing electrons so that each atom gets a stable outer-shell arrangement (duplet for hydrogen; octet for most others at O-Level).

Dot-and-cross diagrams for hydrogen, oxygen, nitrogen, water, methane and carbon dioxideThe diagrams show shared electron pairs and lone pairs. Methane has four single carbon-hydrogen bonds. Carbon dioxide has two carbon-oxygen double bonds.H₂: one single bondHH•×one shared pairO₂: one double bondOO•×•×••••××××N₂: one triple bondNN•×•×•×••××H₂O: two single bondsHOHו•×••••Dots and crosses show origin; both are electrons.CH₄: four single bondsCHHHH•×•×•×•×CO₂: two double bondsCOOוו•×•×××××××××
Dot-and-cross covalent bonding: every shared pair contains one electron from each bonded atom. Lone pairs remain outside the overlap.

B. Single, Double, Triple Bonds (What the Words Mean)

Bond typeShared pairsExample
Single1H₂, HCl, CH₄
Double2O₂, CO₂ (two C = O double bonds)
Triple3N₂

C. Simple Molecular vs Giant Covalent (Exam Comparison)

Most covalent compounds are simple molecular. This table is how you score comparison marks.

FeatureSimple molecular (most covalent)Giant covalent (some covalent)
StructureSmall moleculesGiant network of atoms
Melting/boiling pointLowVery high
WhyWeak forces between moleculesMany strong covalent bonds to break
Electrical conductivityDoes not conductUsually does not conduct (graphite is an exception)
ExamplesCH₄, CO₂, H₂ODiamond (C), graphite (C), SiO₂
Key term: intermolecular forces

Intermolecular forces are weak forces of attraction between molecules (not the covalent bonds within a molecule). They are what you overcome when a simple molecular substance melts or boils.

4. Common Mistakes

  • Writing “covalent bonding = transfer of electrons” (wrong). Covalent = sharing.
  • Using “intermolecular forces” when you mean “covalent bonds” (or vice versa).
  • Saying covalent substances never have high melting points (wrong). Giant covalent structures have very high melting points.
  • Saying covalent substances conduct when molten “like ionic” (wrong). Simple covalent molecules do not form ions.
  • Not stating the number of shared pairs for double/triple bonds.

5. Exam Tips

Match the wording to the question

For formation, state that two atoms share a pair of electrons. If asked what holds the atoms together, add that the shared pair is attracted to both nuclei.

Melting point explanation trap

For simple molecular substances, melting/boiling is about weak intermolecular forces, not breaking covalent bonds.

6. Worked Examples

Modelled example 1

Describe Covalent Bonding (Methane)

Core

Problem

Describe the bonding in a methane molecule, CH₄.
Study the worked solution

  1. Count valence electrons

    Method

    Identify the electrons available for bonding.

    Reason

    Carbon has four valence electrons; each hydrogen has one.

    Working

    C: 4 valence electrons; 4 ×H: one each.

  2. Form shared pairs

    Method

    Share one pair between carbon and each of four hydrogen atoms.

    Reason

    Each shared pair is attracted to both nuclei and forms one single covalent bond.

    Working

    Four C-H shared pairs → four single covalent bonds.

  3. Check outer shells

    Method

    Count each shared pair for both bonded atoms.

    Reason

    Carbon then has an octet and each hydrogen has a duplet.

    Working

    CH₄ is a simple molecule with four C-H covalent bonds.

Guided practice 2

Identify Bond Type From the Elements

About 5 min

Problem

Predict the bonding type in (a) MgO and (b) CO₂. Give a particle-level reason for each.

Classify each element before choosing the bond

MgO bonding
CO2 bonding

Hints

Hint 1: classify the elements
Magnesium is a metal; carbon and oxygen are non-metals.
Hint 2: connect element types to electrons
Metal plus non-metal usually transfers electrons; non-metal plus non-metal shares them.
View solution step by step

  1. Analyse magnesium oxide

    Method

    Apply the metal–non-metal pattern.

    Reason

    Electrons transfer from magnesium to oxygen, forming oppositely charged ions.

    Working

    MgO has ionic bonding.

  2. Analyse carbon dioxide

    Method

    Apply the non-metal–non-metal pattern.

    Reason

    Carbon and oxygen atoms share electron pairs rather than forming a metal cation.

    Working

    CO₂ has covalent bonding.

Common misconception 3

Error Analysis (Fix the Student)

Find and correct the mistake

Learner response

A student writes: “Methane has a low boiling point because covalent bonds are weak.” Locate the first error and correct the explanation.

Identify what boiling overcomes

Force overcome during boiling
Covalent bonds in methane

View solution step by step

  1. Locate the first error

    Method

    Reject the claim that C-H covalent bonds are weak.

    Reason

    Boiling does not decompose methane molecules, so the strong bonds within each molecule remain intact.

    Working

    CH₄(l) → CH₄(g) keeps each methane molecule intact.

  2. Name the relevant attraction

    Method

    Focus on forces between separate methane molecules.

    Reason

    Only weak intermolecular forces need to be overcome to separate the molecules.

    Working

    Weak intermolecular forces → little energy needed for boiling.

  3. Write the correction

    Working

    Methane has a low boiling point because the intermolecular forces between its molecules are weak, not because its covalent bonds are weak.

Examiner practice 4

Property Comparison (Ionic vs Covalent)

4 marks

Examination question

Explain why sodium chloride, NaCl, has a much higher melting point than methane, CH₄. [4 marks]

Compare structure, force and energy

View solution step by step

  1. Explain sodium chloride

    2 marks

    Method

    Identify its giant ionic lattice and strong attractions.

    Reason

    Much energy is required to overcome electrostatic attraction between oppositely charged ions.

    Working

    Giant lattice + strong ionic attractions → high melting point.

  2. Explain methane

    2 marks

    Method

    Identify its simple molecular structure.

    Reason

    Only weak intermolecular forces between methane molecules are overcome during melting; the C-H bonds remain intact.

    Working

    Less energy is needed, so methane has the lower melting point.

Challenge 5

Double vs Single Bonds (CO2)

Minimal support

Electron-count transfer

Carbon has four valence electrons and each oxygen has six. Deduce why the structure of carbon dioxide is O = C = O rather than using two single C-O bonds.

Count shared pairs needed for each octet

Pairs shared with each oxygen
Each C-O bond

Hints

Hint 1: start from oxygen
Each oxygen begins with six valence electrons and needs access to two more for an octet.
Hint 2: translate pairs into bond type
One shared pair is a single bond; two shared pairs between the same atoms form a double bond.
View solution step by step

  1. Determine each oxygen's need

    Method

    Compare six valence electrons with an octet.

    Reason

    Each oxygen needs access to two additional electrons.

    Working

    8-6 = 2 electrons needed by each O atom.

  2. Determine carbon's need

    Method

    Distribute carbon’s four bonding electrons across two oxygen atoms.

    Reason

    Sharing two pairs with each oxygen gives all three atoms an octet.

    Working

    2 shared pairs per C-O connection × 2 oxygen atoms.

  3. State the bond multiplicity

    Working

    Two shared pairs form each C=O double bond, giving O = C = O.

7. Mind Stretchers

Mind stretcher 1: Data Interpretation (Which Is Giant Covalent?)Extension

Question: Substance A melts at -183°C. Substance B melts at 1610°C. Which is more likely to be a giant covalent structure? Explain.

Show Answer

Very high melting point suggests many strong bonds must be broken → giant covalent.

Final: Substance B is more likely to be giant covalent.

Mind stretcher 2: Conductivity Trap (Graphite Exception)Extension

Question: A student says: “All covalent substances do not conduct electricity.” Explain why this statement is unsafe.

Show Answer

Most covalent substances do not conduct because there are no mobile charged particles.

Graphite is an exception: it has delocalised electrons that can move through the layers and carry charge.

8. Quiz

Quiz time

Ready to check your understanding? Try the interactive quiz, then review any questions you missed.

Go to quiz