Shared pairs and simple covalent molecules
Build dot-and-cross models and distinguish bonds inside molecules from attractions between them.
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Know the outer-electron counts of H, C, N, O and Cl.
- Distinguish shared electrons from electron transfer.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many electrons fill hydrogen's first shell?
What you will learn
- Represent single covalent bonds in the required simple molecules.
- Describe the melting and conductivity properties of simple molecular substances.
One bond is one shared pair
A covalent bond is a shared pair of electrons between two atoms. Each atom contributes to the pair, and the shared electrons count toward each atom's outer-shell arrangement. Hydrogen needs two electrons in its first shell; the common second-period examples here use eight in their outer shell. Use dots and crosses to track electron origin, not different electron charges.
For H₂, show one shared pair and no lone pairs. Cl₂ has one shared pair and three lone pairs on each chlorine. HCl has one shared pair and three lone pairs on chlorine. H₂O has two O–H shared pairs and two lone pairs on oxygen. NH₃ has three N–H shared pairs and one lone pair on nitrogen. CH₄ has four C–H shared pairs and no lone pairs on carbon. A line in a displayed formula represents a shared pair; a full dot-and-cross diagram also shows unshared outer electrons.
The whole substance has many molecules
Simple molecular substances generally have low melting and boiling points compared with giant structures. Melting separates molecules from one another rather than breaking their covalent bonds into individual atoms. Distinguish a molecule's strong internal bonds from the attractions between separate molecules.
A typical pure simple molecular substance does not conduct electricity because it lacks mobile charged particles. A covalent bond contains electrons, but those electrons are not a freely moving sea throughout the sample. Some substances form ions after reacting with water: hydrogen chloride gas is molecular, while aqueous hydrochloric acid contains ions and conducts. Always specify the sample and state.
Sources for this lesson
Pause and explain
What does one covalent bond represent?
Worked example
Describe the outer-shell dot-and-cross diagram for water, H₂O.
Show the worked solution
- Oxygen brings six outer electrons and each hydrogen brings one.
- One shared pair forms each O–H bond, giving two shared pairs.
- Four of oxygen's electrons remain as two lone pairs; oxygen counts eight outer electrons and each hydrogen counts two.
Answer Two O–H shared pairs and two lone pairs on oxygen.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw outer-shell diagrams for H₂ and Cl₂. Count the shared and lone pairs.
Give me a hint
Each molecule has a single bond, but only chlorine has extra unshared outer electrons.
Compare my reasoning
- H₂ has one shared pair, filling each hydrogen's first shell.
- Cl₂ has one shared pair, giving each chlorine eight counted outer electrons.
- Each chlorine also has three lone pairs; H₂ has none.
Both have one shared pair; Cl₂ has three lone pairs per chlorine, H₂ none.
Look for these in your work
- Shared pairs contain one electron from each atom.
- Unshared chlorine electrons are included.
Describe full outer-shell diagrams for CH₄ and HCl, including lone pairs and the shell count for hydrogen.
Give me a hint
Carbon makes four single bonds; chlorine makes one and keeps six unshared electrons.
Compare my reasoning
- CH₄ has four C–H shared pairs and no carbon lone pairs.
- HCl has one shared pair and three lone pairs on chlorine.
- Each hydrogen counts two electrons; carbon and chlorine count eight.
CH₄: four shared pairs. HCl: one shared pair and three chlorine lone pairs.
Look for these in your work
- Both molecules' complete outer electrons are accounted for.
- Hydrogen is given a duet, not an octet.
A pure molecular solid melts easily and does not conduct when liquid. A student says its molecules lose all bonds on melting. Correct this explanation.
Give me a hint
Separate attractions between molecules from bonds within them.
Compare my reasoning
- Melting overcomes attractions between molecules.
- Covalent bonds within the molecules remain intact in this physical change.
- The liquid still lacks mobile ions or delocalised electrons, explaining poor conductivity.
Molecules stay intact; their separation changes, and the liquid lacks mobile charge carriers.
Look for these in your work
- Internal bonds are distinguished from intermolecular attractions.
- Conductivity is explained by charge carriers.
Common mistakes
- Forgetting lone pairs when drawing a full electron diagram.
- Saying molecules melt because all their covalent bonds break.
What can you explain now?
How many shared pairs and nitrogen lone pairs are in NH₃?
Compare with the explanation
Three shared pairs and one lone pair on nitrogen
Nitrogen brings five outer electrons; three are shared with hydrogen and the remaining two form one lone pair.
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Make it stick
Draw H₂, Cl₂, H₂O, CH₄, NH₃ and HCl without notes. Check every outer electron, then explain why a molecular solid can melt without breaking its molecules apart.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.