The shape of the lesson
By the end of the lesson, learners can
- Sharing a pair: Build each molecule in the simulator.
- Double and triple bonds: Extended.
- Properties of simple molecules: Methane, water, ammonia and chlorine are all made of small separate molecules, and that single fact explains their properties.
- One bond = one shared pair = two electrons.
- Hydrogen ends with 2 outer electrons; other atoms with 8.
- Lone pairs are outer electrons not used in bonds.
- Melting or boiling overcomes weak intermolecular forces, not covalent bonds.
One picture every idea returns to
A covalent bond is formed when a pair of electrons is shared between two atoms, usually non-metals. By sharing, each atom reaches a noble-gas configuration. Simple molecular substances have low melting and boiling points and are poor conductors of electricity. (Extended) Their melting points are low because only the weak intermolecular forces between molecules are overcome; the strong covalent bonds inside the molecules do not break.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Share a pair.. Two non-metal atoms both want electrons, so neither can simply take them. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Sharing a pair | Build each molecule in the simulator. Project tab 2 of the lab (Chemical bonding lab, dot-and-cross) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Double and triple bonds | Extended. | Pairs |
| 21 to 30 min | Teach and check: Properties of simple molecules | Methane, water, ammonia and chlorine are all made of small separate molecules, and that single fact explains their properties. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Properties of simple molecules, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Each bond line stands for one shared pair, two electrons. | A double bond is two pairs, four electrons; a triple bond three pairs, six electrons. Drawing CO 2 with single bonds leaves each oxygen with only seven outer electrons, so always check that every atom other than hydrogen ends with eight. |
| When water boils, the O-H covalent bonds do not break; the molecules simply move apart and the steam is still H 2 O. | An answer saying 'covalent bonds are weak' or 'covalent bonds break on boiling' loses the mark: it is the intermolecular forces that are weak. |
Simple molecules and covalent bonds: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by a covalent bond and explain why atoms form one.
Describe the dot-and-cross diagram for ammonia, NH3. State the number of bonding pairs and lone pairs and the number of outer electrons around nitrogen.
(Extended) For ethene, C2H4, state the number of shared pairs of electrons in the molecule and the total number of bonding electrons. Show that each carbon atom has eight outer electrons.
(Extended) Methanol has the formula CH3OH. State the number of single covalent bonds in the molecule and the number of lone pairs on the oxygen atom.
(Extended) Chlorine, Cl2, is a gas at room temperature. Explain why, in terms of structure and bonding.
Explain why simple molecular substances such as methane are poor conductors of electricity.
Answer key and marking notes
- A covalent bond is a pair of electrons shared between two atoms.
- By sharing, each atom gains a full outer shell (noble-gas configuration).
- Three shared pairs, each made of one nitrogen electron and one hydrogen electron (three N-H bonds).
- One lone pair on the nitrogen atom.
- Nitrogen has 3 × 2 + 2 = 8 outer electrons; each hydrogen has 2.
- One C=C double bond (2 shared pairs) and four C-H bonds (4 shared pairs): 6 shared pairs.
- Total bonding electrons = 6 × 2 = 12.
- Each carbon: 4 electrons in the double bond + 2 × 2 in its two C-H bonds = 8.
- Five single bonds: three C-H, one C-O and one O-H.
- Two lone pairs on the oxygen atom.
- Chlorine is made of small simple molecules.
- The intermolecular forces between the molecules are weak, so little energy is needed to overcome them.
- The strong covalent bond within each Cl2 molecule is not broken when chlorine boils.
- The molecules have no overall charge.
- There are no ions or delocalised electrons free to move and carry charge.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
The dot-and-cross diagram shows the outer-shell electrons in a molecule XH2. Which element could X be?
- Aoxygen
- Bnitrogen
- Ccarbon
- Dmagnesium
Covalent bonds usually form between:
- Atwo non-metal atoms
- Ba metal atom and a non-metal atom
- Ctwo metal atoms
- Da noble gas atom and a non-metal atom
In a molecule of tetrachloromethane, CCl4, the carbon atom forms a single covalent bond to each chlorine atom. How many lone (non-bonding) pairs of electrons are there in the whole molecule?
- A12
- B0
- C4
- D3
Which pair of properties is typical of a simple molecular substance?
- Alow melting point and does not conduct electricity
- Bhigh melting point and conducts when molten
- Chigh melting point and conducts when solid
- Dvery hard, with a very high melting point
An oxygen molecule, O2, contains a double bond. How many lone (non-bonding) pairs of electrons are on each oxygen atom?
- A2
- B3
- C1
- D4
Which of these molecules contains a triple covalent bond?
- AN2
- BO2
- CCO2
- DC2H4
Chlorine, Cl2, is a gas at room temperature, but iodine, I2, is a solid. Both are made of simple diatomic molecules. What is the best explanation?
- AThe intermolecular forces between the larger I2 molecules are stronger, so more energy is needed to separate them
- BThe covalent bond inside an I2 molecule is stronger than the bond inside a Cl2 molecule
- CIodine is held together by ionic bonds
- DIodine has a giant covalent structure
How many electrons in total are shared between atoms in one molecule of carbon dioxide, CO2?
- A8
- B4
- C16
- D2
Quiz key
X has 6 dots: 2 in bonding pairs and 4 in two lone pairs, so X has six outer electrons and is in Group VI, like oxygen (the molecule is water). Nitrogen has five outer electrons and forms three bonds (NH3); carbon has four and forms four bonds (CH4). Magnesium is a metal: it forms ions, not covalent bonds with hydrogen, and it has only two outer electrons.
Non-metal atoms both need to gain electrons to fill their outer shells, so they share pairs of electrons instead of transferring them. A metal and a non-metal form ions (ionic bonding); metal atoms together form metallic bonding. Noble gases already have full outer shells, so they do not normally form bonds.
Carbon uses all four outer electrons in the four C-Cl bonds, so it has no lone pairs. Each chlorine (2,8,7) uses one electron in its bond and keeps six, which is three lone pairs. 4 chlorine atoms × 3 lone pairs = 12. 0 counts only the carbon, 3 counts only one chlorine, and 4 is the number of bonding pairs.
Only the weak forces between molecules have to be overcome to melt a simple molecular substance, so its melting point is low. The molecules are neutral, with no ions or free electrons, so it does not conduct. Conducting when molten is typical of ionic compounds, conducting when solid is typical of metals and graphite, and very hard with a very high melting point describes giant covalent structures such as diamond.
A double bond is two shared pairs, and each oxygen contributes one electron to each pair, so each atom uses 2 of its 6 outer electrons in bonding. The remaining 4 form 2 lone pairs, and each atom then has 4 shared + 4 non-bonding = 8 outer electrons. 3 lone pairs would be correct for a single bond, which would leave each oxygen with only 7 outer electrons. 4 counts the shared electrons instead of lone pairs.
Nitrogen (2,5) needs three more electrons, so the two atoms in N2 share three pairs: a triple bond. O2 has one double bond, CO2 has two C=O double bonds, and C2H4 (ethene) has one C=C double bond and four single C-H bonds.
Melting and boiling a simple molecular substance only separates the molecules; the covalent bonds inside them do not break. I2 molecules are larger than Cl2 molecules, so the intermolecular forces between them are stronger, and iodine needs a higher temperature to melt. The I-I bond strength is irrelevant to the change of state (and it is actually weaker). Iodine is a non-metal element made of molecules, so it is neither ionic nor giant covalent.
CO2 has two C=O double bonds. Each double bond is 2 shared pairs, so there are 4 shared pairs, which is 4 × 2 = 8 electrons. 4 is the number of shared pairs, not electrons. 16 is the total number of outer electrons (4 + 6 + 6), which includes the four lone pairs on the oxygen atoms. 2 is the number of double bonds, not the number of shared electrons.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.