This lesson introduces the second major bonding model in the unit. Instead of electron transfer and ions, students now see atoms sharing electrons to achieve more stable outer shells, and use that model to explain common molecular substances and their properties.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Write your best idea before you read the lesson. The key move here is different from ionic bonding.
Think about how molecules behave in gases versus liquids, and what keeps them close together in a liquid.
This lesson matters because students often overuse the ionic model. Not every stable compound forms by electron transfer.
When two non-metal atoms react, neither may be likely to lose electrons completely to the other. Instead, the atoms can share electrons. Each shared electron pair counts for both atoms, helping each one reach a more stable outer-shell arrangement. That shared pair forms a covalent bond.
Stage 5 students should be able to recognise and discuss simple molecular substances. These examples show that covalent bonding is common in everyday materials and substances, not just in textbook diagrams.
Two hydrogen atoms share one pair of electrons.
Oxygen shares electrons with two hydrogen atoms.
Carbon shares electrons with two oxygen atoms.
At this depth, the important idea is that these substances are made of neutral molecules. Students do not need advanced bond-angle or shape analysis here. The focus is the bonding logic and the broad effect on properties.
Many simple molecular substances have lower melting and boiling points than ionic substances. Many are gases or liquids at room temperature, and they usually do not conduct electricity well because they do not contain free-moving charged ions like molten ionic substances do.
A strong materials student should now be able to compare the two bonding models without mixing them up. Ionic bonding involves transfer and attraction between ions. Covalent bonding involves sharing between atoms, often forming molecules. The kind of structure formed helps explain why the properties differ.
Wrong: Covalent bonds are weak because simple molecular substances have low melting points.
Right: Covalent bonds within molecules are strong. The low melting points are due to weak forces between molecules, not weak covalent bonds.
Wrong: Covalent substances contain ions that can move.
Right: Covalent molecular substances are made of neutral molecules. They do not contain free-moving ions like molten ionic substances do.
Right: In a covalent bond, both atoms count the shared pair toward their outer shell. Each atom still has full access to both electrons.
Right: Many simple molecular substances are gases or liquids, but this is not true of all covalent structures. The state depends on the substance.
Select a molecule to see how atoms share electrons in covalent bonds.
Covalent bonding happens when atoms share electrons. The shared electrons help both atoms move toward a more stable outer shell.
Simple molecular substances such as H2, H2O and CO2 are made of neutral molecules held together by covalent bonds.
Many simple molecular substances have relatively low melting and boiling points and do not conduct electricity well.
Covalent bonding involves sharing electrons, while ionic bonding involves electron transfer and attraction between oppositely charged ions.
For each example, decide whether the lesson is describing covalent bonding or ionic bonding, then explain why.
a. A bond formed between Na+ and Cl-
b. Two hydrogen atoms held together in H2
c. A substance made of neutral molecules
d. A substance that conducts when molten because ions can move
Choose one of these molecules: H2, H2O or CO2. Explain what is being shared and why the substance is considered covalent.
Claim: State which molecule you chose and whether it is covalent.
Evidence: Describe what is shared and which atoms are involved.
Reasoning: Explain why sharing electrons makes this a covalent molecular substance rather than an ionic compound.
1. Which statement best describes covalent bonding?
2. Which substance from this lesson is a simple molecular substance?
3. Why do many simple molecular substances not conduct electricity well?
4. Which comparison is correct?
5. Which conclusion best links covalent structure to a real-world use?
Explain how covalent bonding is different from ionic bonding. 1 mark for stating covalent bonding involves sharing electrons. 1 mark for stating ionic bonding involves electron transfer and attraction. 1 mark for clearly identifying the main difference.
Use water, H2O, to explain why it is described as a covalent molecular substance. 1 mark for identifying hydrogen and oxygen atoms in water. 1 mark for stating that electrons are shared. 1 mark for explaining that water is made of neutral molecules. 1 mark for concluding it is a covalent molecular substance.
Choose one simple molecular substance from the lesson and explain one property and one use that fit that substance. 1 mark for naming a simple molecular substance. 1 mark for stating one property. 1 mark for describing a relevant use. 1 mark for linking the property to the use.
Return to the opening question. Can you now explain how two non-metal atoms can both move toward stability by sharing electrons rather than forming ions?
1: C. Covalent bonding is based on shared electrons.
2: A. Water is a simple molecular covalent substance.
3: D. Many simple molecular substances lack free-moving charged particles.
4: B. That is the correct comparison between the two bonding models.
5: A. This correctly links a covalent substance to a property and a use.
Sample answer: Covalent bonding happens when atoms share electrons. Ionic bonding is the attraction between oppositely charged ions after electrons have been transferred. The main difference is that covalent bonding uses sharing, while ionic bonding depends on ion formation and attraction.
1 mark for stating covalent bonding involves sharing electrons. 1 mark for stating ionic bonding involves electron transfer and attraction. 1 mark for clearly identifying the main difference.
Sample answer: Water contains hydrogen atoms and an oxygen atom. The atoms share electrons, so covalent bonds form between oxygen and hydrogen. This means water is made of neutral molecules rather than ions. So it is described as a covalent molecular substance.
1 mark for identifying hydrogen and oxygen atoms in water. 1 mark for stating that electrons are shared. 1 mark for explaining that water is made of neutral molecules. 1 mark for concluding it is a covalent molecular substance.
Sample answer: One example is carbon dioxide. One property is that it is a gas at room temperature. A relevant use is in fire extinguishers or carbonated drinks. This fits because carbon dioxide is a simple molecular substance with properties that suit those uses.
1 mark for naming a simple molecular substance. 1 mark for stating one property. 1 mark for describing a relevant use. 1 mark for linking the property to the use.
Covalent bonds form when atoms share electrons so both atoms count the shared pair.
Simple molecular substances such as H2, H2O and CO2 are made of neutral molecules.
Many simple molecular substances show relatively low melting and boiling points and poor electrical conductivity.
Next lesson adds metallic bonding and then compares all three bonding models together.