The shape of the lesson
By the end of the lesson, learners can
- Sharing pairs: Build H 2, O 2 and N 2 in the simulator, adding electrons one at a time.
- One atom gives both: A dative bond needs a donor with a lone pair and an acceptor with an empty orbital.
- More, or fewer, than eight: The octet rule works for most Period 2 molecules, but not all.
- Covalent: attraction of two nuclei to a shared pair.
- Dative: both electrons from one atom; arrow from donor to acceptor.
- Donor needs a lone pair; acceptor needs an empty orbital.
- Period 3 atoms can exceed eight; B in BF3 has six.
One picture every idea returns to
A covalent bond is the electrostatic attraction between two nuclei and a shared pair of electrons. Atoms can share one, two or three pairs to form single, double and triple bonds. In a dative covalent (coordinate) bond both electrons of the shared pair come from the same atom, which must have a lone pair, and the other atom must have an empty orbital to receive it; it is drawn as an arrow from donor to acceptor. Once formed, a dative bond is identical to any other covalent bond. Period 3 elements can use d orbitals to hold more than eight outer electrons (PCl5, SF6), while boron in BF3 has only six.
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, A pair, shared.. Two non-metal atoms cannot both win an electron, so they share. 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 pairs | Build H 2, O 2 and N 2 in the simulator, adding electrons one at a time. 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: One atom gives both | A dative bond needs a donor with a lone pair and an acceptor with an empty orbital. | Pairs |
| 21 to 30 min | Teach and check: More, or fewer, than eight | The octet rule works for most Period 2 molecules, but not all. | Jigsaw |
| 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, 14 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 More, or fewer, than eight, 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.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
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 |
|---|---|
| When you draw a dative bond, the arrow points from the atom that donates the lone pair to the atom that accepts it. | In a dot-and-cross diagram, show both electrons of the dative pair with the donor's symbol. And remember the finished bond is not weaker or different: all four bonds in NH 4 + have the same length and strength. |
Covalent and dative covalent bonding: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Define (a) a covalent bond and (b) a dative covalent bond.
Describe how the ammonium ion forms from ammonia and a hydrogen ion, and explain why all four N–H bonds in the ion are the same length.
At lower temperatures aluminium chloride vapour contains Al2Cl6 molecules. Explain how two AlCl3 units join together.
State the number of bonding pairs and lone pairs around the central atom in (a) PCl5 and (b) BF3.
Explain why phosphorus can form PCl5 but nitrogen cannot form NCl5.
Carbon monoxide contains a triple bond. Explain how the triple bond is made up, given that carbon has four and oxygen six outer electrons.
Answer key and marking notes
- (a) The electrostatic attraction between two nuclei and a shared pair of electrons.
- (b) A covalent bond in which both electrons of the shared pair are supplied by one of the atoms.
- The nitrogen in NH3 has a lone pair; H+ has an empty 1s orbital.
- The lone pair is donated to H+, forming a dative covalent bond.
- Once formed, the dative bond is a shared pair like the others, so all four N–H bonds are identical.
- Each Al in AlCl3 has only six outer electrons, so it has an empty orbital.
- A chlorine atom in each AlCl3 has lone pairs.
- A lone pair from a Cl of one unit forms a dative bond to the Al of the other, and vice versa: two dative bonds.
- (a) PCl5: 5 bonding pairs, 0 lone pairs (10 electrons).
- (b) BF3: 3 bonding pairs, 0 lone pairs (6 electrons).
- Phosphorus is in Period 3 and has empty 3d orbitals, so it can hold ten electrons in its outer shell.
- Nitrogen is in Period 2; its second shell can hold at most eight electrons, so it cannot form five bonds.
- Two of the pairs are ordinary covalent bonds, one electron from C and one from O in each.
- The third pair is a dative bond: a lone pair on O is donated to C, so both atoms reach eight outer electrons.
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.
Which statement describes a single covalent bond?
- Aa pair of electrons shared between two atoms, held by the attraction of both nuclei for the shared pair
- Bone electron transferred from one atom to another
- Cthe attraction between two nuclei that have lost their outer electrons
- Done electron shared between two atoms
What is a dative covalent (co-ordinate) bond?
- Aa shared pair of electrons in which both electrons come from the same atom
- Ba shared pair in which one electron comes from each atom
- Ca bond in which one atom gives an electron completely to the other
- Da covalent bond between two identical atoms
Which species contains a dative covalent bond?
- AH3O+
- BH2O
- COH−
- DH2O2
How many electrons are shared between the two atoms in a nitrogen molecule, N2?
- A6
- B3
- C2
- D10
In which molecule does the central atom have more than eight electrons in its outer shell?
- ASF6
- BBF3
- CNH3
- DCCl4
Ammonia reacts with boron trifluoride to form the compound H3NBF3. Which statement describes the new bond?
- Aa dative bond in which the nitrogen lone pair is donated into an empty orbital on boron, which had only six outer electrons
- Ba dative bond in which boron donates a lone pair to nitrogen
- Can ionic bond formed when nitrogen transfers an electron to boron
- Da dative bond in which a fluorine lone pair is donated to nitrogen
In the vapour, aluminium chloride exists as Al2Cl6 molecules, in which two AlCl3 units are joined. How many dative bonds are there in one Al2Cl6 molecule?
- A2
- B1
- C4
- D6
In carbon monoxide both atoms achieve an octet. Which describes the bonding between the carbon and oxygen atoms?
- Aa triple bond made of two ordinary covalent bonds and one dative bond from oxygen to carbon
- Ba triple bond made of two ordinary covalent bonds and one dative bond from carbon to oxygen
- Ca double bond, with carbon having only six outer electrons
- Dan ionic bond between C2+ and O2− ions
Quiz key
A single covalent bond is one shared pair (two electrons), usually one from each atom, and it holds the atoms together because both positive nuclei are attracted to the negative electron pair between them. Transfer of an electron forms ions and ionic bonding. Two bare nuclei would repel; the bond depends on the electrons between them. One shared electron is only half of the pair; a single bond contains two electrons.
In a dative bond one atom supplies a lone pair and the other atom has an empty orbital to accept it, so both shared electrons come from the donor atom. One electron from each atom is an ordinary covalent bond. Complete transfer of an electron forms ions, not a covalent bond. A bond between identical atoms (for example in Cl2) is a non-polar covalent bond, not necessarily dative.
In H3O+ an H+ ion, which has no electrons, bonds to one of the lone pairs on the oxygen of a water molecule, so that O–H bond is dative (once formed, all three O–H bonds are identical). In H2O oxygen forms two ordinary bonds using its two unpaired electrons. OH− has one ordinary O–H bond; the extra electron gives the charge. In H2O2 every bond (O–H and O–O) uses one electron from each atom.
Each N atom shares three of its electrons, forming a triple bond of three shared pairs, which is 6 electrons; each N also keeps one lone pair. 3 is the number of bonds (pairs), not the number of electrons. 2 electrons is a single bond, which would leave each N with only six outer electrons. 10 is the total number of outer electrons in the molecule, which includes the two lone pairs that are not shared.
In SF6 sulfur forms six bonding pairs, so it has 12 electrons in its outer shell; this expanded octet is possible for a Period 3 element (other examples are PCl5 with 10 and SO2 with 10). In BF3 boron has only 6 outer electrons, so it is electron-deficient, the opposite case. In NH3 and CCl4 the central atom has exactly eight (three bonding pairs and a lone pair, and four bonding pairs); Period 2 atoms cannot exceed eight.
Nitrogen in NH3 has a lone pair, and boron in BF3 has only three bonding pairs (six outer electrons), leaving an empty orbital. The lone pair is donated from N to B, giving boron a full octet. Boron has no lone pair to donate. No electron is transferred, so no ions are formed; the product is a molecule. Fluorine has lone pairs, but nitrogen already has an octet and cannot accept another pair.
Each Al in AlCl3 is electron-deficient with six outer electrons. In the dimer, a chlorine atom of each unit donates a lone pair to the aluminium atom of the other unit, forming two dative bonds, and both Al atoms reach an octet. The two bridging Cl atoms each end up bonded to two Al atoms. One dative bond would complete the octet of only one Al atom. 4 counts all four bonds to the two bridging chlorines, but two of these are ordinary covalent bonds. 6 is the number of chlorine atoms, not the number of dative bonds.
Two ordinary shared pairs give oxygen an octet but leave carbon with only six electrons. Oxygen then donates one of its lone pairs to carbon, giving a triple bond of three shared pairs in which both atoms have eight outer electrons and each keeps one lone pair. Carbon cannot donate to oxygen because oxygen already has an octet, and carbon is the atom that is short of electrons. A plain double bond is the structure that leaves carbon without an octet, which the question rules out. Carbon monoxide is a covalent molecular gas, not an ionic compound.
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.