The shape of the lesson
By the end of the lesson, learners can
- Transfer, then attract: Build the sodium chloride lattice in the simulator and rotate it.
- What makes it strong?: The force between two charges grows with the size of the charges and shrinks as they move apart.
- Properties from the lattice: Nearly every property of an ionic compound follows from strong, non-directional attraction between charged particles that are fixed in place in the solid.
- Definition: electrostatic attraction between oppositely charged ions.
- Giant lattice: attraction in all directions, no molecules.
- Strength: rises with ionic charge, falls with ionic radius.
- Conducts only when the ions can move: molten or in solution.
One picture every idea returns to
Ionic bonding is the electrostatic attraction between oppositely charged ions. The ions form when electrons transfer, usually from a metal to a non-metal, leaving each with a stable (often noble gas) configuration. The ions arrange themselves in a giant lattice in which every ion is surrounded by ions of opposite charge, so the attraction acts in all directions. The bonding is stronger when the ions carry larger charges and when they are smaller, because the charges get closer together; that is why MgO melts at about 2850 °C and NaCl at 801 °C.
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, Opposites in a lattice.. When a metal hands its outer electrons to a non-metal, the ions that result do not pair off. 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: Transfer, then attract | Build the sodium chloride lattice in the simulator and rotate it. Project tab 3 of the lab (Chemical bonding lab, ionic lattice) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: What makes it strong? | The force between two charges grows with the size of the charges and shrinks as they move apart. | Pairs |
| 21 to 30 min | Teach and check: Properties from the lattice | Nearly every property of an ionic compound follows from strong, non-directional attraction between charged particles that are fixed in place in the solid. | Think-Pair-Share |
| 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 Properties from the lattice, 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.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
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 |
|---|---|
| Ionic solids conduct nothing because the ions cannot move, and it is ions, not electrons, that carry the current in a melt or solution. | Writing “free electrons” for molten NaCl loses the mark. Also avoid the word “molecule” for an ionic compound: NaCl is a formula unit in a giant lattice. |
Ionic bonding: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Define ionic bonding.
Describe, in terms of electrons, how magnesium and oxygen form magnesium oxide. Give the full electron configurations of both ions.
Write the formulae of the ionic compounds formed between (a) aluminium and oxygen and (b) potassium and sulfur.
Magnesium oxide melts at 2852 °C but sodium chloride melts at 801 °C. Explain the difference.
Explain why sodium chloride does not conduct electricity as a solid but does when molten or dissolved in water.
Explain why an ionic crystal shatters when struck with a hammer.
Answer key and marking notes
The electrostatic attraction between oppositely charged ions.
- Each magnesium atom transfers its two 3s electrons to an oxygen atom.
- Mg2+: 1s2 2s2 2p6.
- O2−: 1s2 2s2 2p6. Both ions have the configuration of neon.
- (a) Al3+ and O2−: Al2O3, total charge 2 × (+3) + 3 × (−2) = 0.
- (b) K+ and S2−: K2S.
- Both are giant ionic lattices, so melting means overcoming the electrostatic attraction between ions.
- Mg2+ and O2− carry double the charges of Na+ and Cl−, and they are smaller, so the ions are closer.
- The attraction in MgO is therefore much stronger and needs more energy to overcome.
- Conduction needs mobile charged particles.
- In the solid, the ions are held in fixed positions in the lattice and cannot move.
- When molten or in solution the lattice breaks up and the ions are free to move and carry charge.
- The blow displaces one layer of ions relative to the next.
- Ions of the same charge are brought next to each other; they repel and the crystal splits along that plane.
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 an ionic bond?
- Athe electrostatic attraction between oppositely charged ions
- Ba shared pair of electrons between a metal atom and a non-metal atom
- Cthe attraction between positive metal ions and a sea of delocalised electrons
- Dthe weak attraction between neighbouring molecules of an ionic compound
Which describes the structure of solid sodium chloride?
- Aa giant lattice in which each Na+ ion is surrounded by six Cl− ions
- Bsimple NaCl molecules held together by weak intermolecular forces
- Ca giant lattice in which each Na+ ion is surrounded by eight Cl− ions
- Dseparate pairs of ions, each Na+ joined to one Cl− by a single ionic bond
Magnesium oxide melts at 2852 °C, but sodium chloride melts at 801 °C. What is the main reason?
- AMg2+ and O2− have larger charges and are smaller than Na+ and Cl−, so the attraction between the ions is stronger
- Bmagnesium oxide has a larger relative formula mass than sodium chloride
- Cmagnesium oxide has covalent bonds, which are stronger than ionic bonds
- Dmagnesium oxide contains more ions in each formula unit than sodium chloride
Aluminium forms Al3+ ions and oxygen forms O2− ions. What is the formula of aluminium oxide?
- AAl2O3
- BAlO
- CAl3O2
- DAlO3
Solid sodium chloride does not conduct electricity, but molten sodium chloride does. Why?
- Ain the solid the ions are held in fixed positions; in the melt the ions are free to move and carry charge
- Bthe solid has no delocalised electrons, but melting releases electrons that carry the current
- Cions are only formed when the solid melts
- Dthe covalent bonds in the solid break on melting, releasing charged atoms
The melting points of the sodium halides decrease in the order NaF (993 °C) > NaCl (801 °C) > NaBr (747 °C) > NaI (661 °C). Which is the best explanation?
- Athe halide ion radius increases, so the ion centres are further apart and the electrostatic attraction is weaker
- Bthe charge on the halide ion increases down the group, so the ions repel each other more
- Clarger halide ions have more electrons, so the instantaneous dipole-induced dipole forces between them decrease
- Dfluorine is the most electronegative halogen, so NaF has the most covalent bonds
Which shows the three compounds in order of increasing strength of ionic bonding? (Ionic radii: K+ 0.138 nm, Ca2+ 0.100 nm, Mg2+ 0.072 nm, Cl− 0.181 nm, O2− 0.140 nm)
- AKCl < CaO < MgO
- BMgO < CaO < KCl
- CCaO < MgO < KCl
- DKCl < MgO < CaO
The diagram shows one layer of the sodium chloride lattice. In this layer each Na+ ion touches four Cl− ions. How many Cl− ions are nearest neighbours of each Na+ ion in the whole three-dimensional lattice?
- A6
- B4
- C8
- D12
Quiz key
After electron transfer the metal forms positive ions and the non-metal negative ions, and the ionic bond is the strong electrostatic attraction between these opposite charges, acting in all directions. A shared pair of electrons is a covalent bond, even when one partner is a metal. Positive ions in a sea of delocalised electrons is metallic bonding. Ionic compounds are giant lattices, not molecules, so there are no intermolecular forces holding molecules together.
Sodium chloride is a giant ionic lattice with a cubic arrangement: each Na+ has six Cl− nearest neighbours and each Cl− has six Na+ (6 : 6 coordination). There are no discrete NaCl molecules; the formula only gives the 1 : 1 ratio of ions. Eight nearest neighbours is the caesium chloride arrangement, which occurs with a much larger cation. Ionic attraction is not directional, so a Na+ ion is not paired with just one Cl−.
Doubling both charges roughly quadruples the attraction between each pair of ions, and Mg2+ and O2− are also smaller than Na+ and Cl−, so the ion centres are closer. Much more energy is needed to break down the lattice. The relative formula mass of MgO (40.3) is actually smaller than that of NaCl (58.5), and mass does not decide the strength of ionic bonding. MgO is ionic, not covalent. Both compounds have one cation and one anion per formula unit.
Two Al3+ ions give +6 and three O2− ions give −6, so Al2O3 is neutral. AlO would have an overall charge of +1. Al3O2 swaps the numbers the wrong way and gives +9 − 4 = +5. AlO3 gives +3 − 6 = −3, so it is not neutral either.
Solid NaCl already contains Na+ and Cl− ions, but they can only vibrate about fixed lattice positions. When the solid melts (or dissolves) the ions become mobile and move to the electrodes, carrying the current. Melting does not release free electrons; the current in the melt is carried by ions. The ions exist in the solid too; melting only frees them to move. There are no covalent bonds between Na and Cl to break.
Every halide ion has a 1− charge, so the difference comes from size: F− is the smallest, so Na+ and F− centres are closest and the attraction is strongest; as the anion gets larger the attraction, and the energy needed to break the lattice, decreases. The charge does not change down the group. Instantaneous dipole-induced dipole forces are intermolecular forces; they are not what holds an ionic lattice together, and in any case they increase with more electrons, which would predict the opposite trend. The large electronegativity difference in NaF makes it more ionic, not more covalent.
KCl has only 1+ and 1− ions, so it has the weakest attraction. CaO and MgO both have 2+ and 2− ions, and Mg2+ is smaller than Ca2+, so the ions in MgO are closer and MgO has the strongest bonding (melting points: KCl 770 °C, CaO 2613 °C, MgO 2852 °C). The second option is the correct order reversed. Putting KCl strongest assumes larger ions attract more, the opposite of the real effect. Putting CaO above MgO wrongly assumes the larger (heavier) calcium ion gives stronger bonding.
In the layer shown each Na+ has four Cl− neighbours, and the layers are stacked so that there is also one Cl− directly above and one directly below, giving 4 + 2 = 6 (6 : 6 coordination). 4 counts only the layer drawn and forgets the third dimension. 8 is the coordination number in caesium chloride, which has a larger cation. 12 is the number of nearest Na+ ions around a Na+ (next-nearest neighbours), not the number of Cl− ions.
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.