The shape of the lesson
By the end of the lesson, learners can
- Build it, electron by electron: Use the dot-and-cross simulator to build H 2 O, CO 2, N 2 and NH 4 + step by step.
- Ionic diagrams: For ionic compounds show the electron transfer, not sharing.
- Tricky covalent cases: These are the diagrams that come up again and again in exam papers.
- Ionic: separate ions, square brackets, charges.
- Covalent: shared pairs in the overlap, lone pairs on their atom.
- Dative: both electrons of the pair use the donor's symbol.
- Total: sum of outer electrons, minus the charge.
One picture every idea returns to
Dot-and-cross diagrams show outer-shell electrons only, using dots for one atom's electrons and crosses for the other's. For an ionic compound, draw each ion separately in square brackets with its charge, showing the electrons transferred. For a covalent molecule, place each shared pair in the overlap between atoms (one dot and one cross), two pairs for a double bond and three for a triple; draw lone pairs on the atom they belong to. In a dative bond both electrons of the shared pair carry the donor's symbol. Check the finish: every atom (except H, B and expanded-octet atoms) has eight outer electrons, and the total matches the number you started with, adjusted for any charge.
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, Draw every electron.. A dot-and-cross diagram is a bookkeeping tool: it shows where each outer electron came from and where it ended up. 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: Build it, electron by electron | Use the dot-and-cross simulator to build H 2 O, CO 2, N 2 and NH 4 + step by step. 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: Ionic diagrams | For ionic compounds show the electron transfer, not sharing. | Pairs |
| 21 to 30 min | Teach and check: Tricky covalent cases | These are the diagrams that come up again and again in exam papers. | Cooperative Concept Mapping |
| 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, 13 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 Tricky covalent cases, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 |
|---|---|
| Count the total before and after. | NH 4 + has 5 + 4 − 1 = 8 outer electrons, which is four pairs and no lone pairs. The most common mistakes are leaving the lone pairs off the outer atoms, forgetting the square brackets and charge on an ion, and drawing a dot and a cross in a dative bond where both electrons should come from the donor. |
Dot-and-cross diagrams: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Draw a dot-and-cross diagram for magnesium chloride, MgCl2. Show outer electrons only.
Draw a dot-and-cross diagram for a molecule of ammonia, NH3, and state the number of lone pairs on nitrogen.
Draw a dot-and-cross diagram for ethene, C2H4.
Draw a dot-and-cross diagram for the oxonium ion, H3O+, showing clearly the dative covalent bond.
Draw a dot-and-cross diagram for carbon monoxide, showing that the molecule contains a dative covalent bond.
A student draws the dot-and-cross diagram of the hydroxide ion, OH−, with oxygen having two lone pairs. Identify the error and give the correct number.
Answer key and marking notes
- [Mg]2+ drawn with no outer electrons (or its full second shell), in square brackets.
- Two separate [Cl]− ions, each with seven dots and one cross (from Mg), in square brackets with a 1− charge.
- Three shared pairs between N and the H atoms, each with one dot (N) and one cross (H).
- One lone pair (two dots) on N, so N has eight outer electrons; one lone pair.
- Two shared pairs between the carbon atoms (a double bond).
- Each carbon also shares one pair with each of two H atoms; no lone pairs; each carbon has eight outer electrons.
- Total outer electrons: 6 + 3 − 1 = 8, so three bonding pairs and one lone pair on O.
- Two O–H pairs with one dot and one cross each; the third O–H pair with both electrons from oxygen (the dative bond).
- Square brackets and a + charge around the whole ion.
- Three shared pairs between C and O: two with one electron from each atom, and one with both electrons from O.
- One lone pair on C and one on O, so each atom has eight outer electrons.
- Outer electrons: 6 + 1 + 1 = 8, four pairs; one is the O–H bonding pair.
- Oxygen should have three lone pairs, one of which includes the extra electron from the charge; the ion needs brackets and a − 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.
In the dot-and-cross diagram of carbon dioxide, O=C=O, how many bonding pairs and how many lone pairs are shown in the whole molecule?
- A4 bonding pairs and 4 lone pairs
- B2 bonding pairs and 4 lone pairs
- C4 bonding pairs and 2 lone pairs
- D2 bonding pairs and 6 lone pairs
In the dot-and-cross diagram of a nitrogen molecule, N2, how many lone pairs are shown on each nitrogen atom?
- A1
- B0
- C2
- D3
Which describes the correct dot-and-cross diagram for magnesium chloride, MgCl2, showing outer electrons only?
- Aone [Mg]2+ ion with no outer electrons shown, and two separate [Cl]− ions, each with eight outer electrons including one cross
- Ba Mg atom sharing one pair of electrons with each of two Cl atoms
- Cone [Mg]2+ ion and one [Cl2]2− ion holding both transferred electrons
- Done [Mg]+ ion and two [Cl]− ions, each with eight outer electrons
A dot-and-cross diagram of the ammonium ion, NH4+, shows nitrogen electrons as crosses and hydrogen electrons as dots. How many of the four N–H bonding pairs are drawn as two crosses?
- A1
- B0
- C3
- D4
Hydrogen cyanide is H–C≡N. In its dot-and-cross diagram, how many bonding pairs and lone pairs are there in the molecule?
- A4 bonding pairs and 1 lone pair
- B2 bonding pairs and 3 lone pairs
- C4 bonding pairs and 2 lone pairs
- D3 bonding pairs and 1 lone pair
In a dot-and-cross diagram of carbon monoxide, carbon electrons are dots and oxygen electrons are crosses, and both atoms have an octet. Which describes the diagram?
- Athree shared pairs (two of them dot-and-cross, one of them two crosses), with one lone pair of dots on C and one lone pair of crosses on O
- Btwo shared dot-and-cross pairs, with one lone pair on C and two lone pairs on O
- Cthree shared pairs, one of them two dots, with one lone pair on C and one lone pair on O
- Dthree shared dot-and-cross pairs, with no lone pairs on either atom
How many lone pairs of electrons are there in total in one Al2Cl6 molecule, in which two chlorine atoms bridge the two aluminium atoms by dative bonds?
- A16
- B18
- C12
- D8
The diagram shows the outer-shell electrons in a molecule A2B, with the atoms arranged A–B–A. Which molecule could it represent?
- ACO2
- BSO2
- CO3
- DBeCl2
Quiz key
Each C=O double bond contains two shared pairs, so there are 4 bonding pairs. Each oxygen has 6 outer electrons, uses 2 in bonding and keeps 4 as 2 lone pairs, so there are 4 lone pairs in total; carbon has none. 2 bonding pairs counts each double bond as one pair. 4 bonding pairs and 2 lone pairs gives each oxygen only one lone pair, leaving it with six outer electrons. 2 bonding pairs and 6 lone pairs describes single bonds with three lone pairs on each O, which would leave carbon with only four outer electrons.
Each N shares three electrons in the triple bond and keeps the other two as one lone pair, so each atom has 3 shared pairs (6 electrons) plus 1 lone pair (2 electrons), an octet. 0 lone pairs would leave each nitrogen with only six outer electrons. 2 lone pairs would give each N 10 electrons with a triple bond, more than an octet for a Period 2 atom. 3 lone pairs is the pattern for a halogen atom with one single bond, as in Cl2.
Magnesium transfers its two outer electrons, one to each chlorine atom. Mg2+ is drawn with an empty outer shell (its full second shell is not shown), and each Cl− has its own seven dots plus one cross from magnesium, drawn in square brackets with its charge. Shared pairs describe covalent bonding, which is not the bonding in MgCl2. The two chloride ions are separate ions, not a Cl2 unit. A 1+ magnesium ion could supply only one electron, so the charges would not balance with two chloride ions.
Three N–H bonds each contain one nitrogen cross and one hydrogen dot. The fourth bond is the dative bond formed when the nitrogen lone pair bonds to H+, which brings no electrons, so that pair is two crosses; the whole ion is drawn in brackets with a + charge. 0 treats all four bonds as ordinary bonds, which would need four hydrogen electrons, but H+ has none. 3 and 4 would mean several hydrogen atoms arrived without electrons, but only one H+ is added to NH3.
The H–C single bond is 1 pair and the C≡N triple bond is 3 pairs, so there are 4 bonding pairs. Carbon uses all four of its electrons in bonds, while nitrogen uses three and keeps one lone pair, so there is 1 lone pair. 2 bonding pairs and 3 lone pairs counts each bond as one pair. 4 bonding pairs and 2 lone pairs wrongly adds a lone pair on carbon, giving it 10 electrons. 3 bonding pairs and 1 lone pair treats the triple bond as only two pairs.
Carbon provides two electrons and oxygen two electrons for two ordinary shared pairs; oxygen then donates a lone pair (two crosses) to form the dative bond. That leaves carbon with 2 dots as one lone pair and oxygen with 2 crosses as one lone pair: 3 + 1 pairs on each atom, an octet each, using all 10 electrons. Two shared pairs with the lone pairs described gives carbon only six outer electrons. A dative pair of two dots would mean carbon donates to oxygen, but oxygen is the atom with electrons to spare. Three dot-and-cross pairs and no lone pairs uses only 6 of the 10 electrons.
Each of the four terminal Cl atoms forms one bond and keeps 3 lone pairs (12). Each bridging Cl forms one ordinary bond and donates one lone pair to the other Al, keeping 2 lone pairs (4). Aluminium has none. Total = 12 + 4 = 16. 18 gives every chlorine three lone pairs and forgets that the two dative bonds use up two of them. 12 counts only the terminal chlorines. 8 is the number of bonding pairs (four Al–Cl terminal bonds plus four bonds to the bridging chlorines), not lone pairs.
B supplies 4 electrons and has no lone pair; each A supplies 6 (two in bonding, four as two lone pairs), and each bond is a double bond. That is O=C=O. In SO2 the central S has 6 outer electrons and keeps a lone pair, which the diagram does not show. In O3 the central oxygen also has 6 electrons and a lone pair. In BeCl2 the bonds are single, each Cl has three lone pairs, and Be has only 2 outer electrons, not 4.
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.