The shape of the lesson
By the end of the lesson, learners can
- Losing and gaining electrons: Pick a metal and a non-metal in the simulator and move the outer electrons across.
- Dot-and-cross diagrams: Draw one atom's electrons as dots and the other's as crosses, so the reader can see where each electron came from.
- Giant ionic lattices: Extended.
- Metal atoms lose outer electrons to form positive ions.
- Non-metal atoms gain electrons to form negative ions.
- Balance the charges to find the formula.
- Molten or aqueous: ions move and carry the current.
One picture every idea returns to
An ion is a charged particle formed when an atom loses or gains electrons. Metal atoms lose their outer electrons to form positive ions; non-metal atoms gain electrons to form negative ions, and both usually end with a noble-gas configuration. An ionic bond is the strong electrostatic attraction between oppositely charged ions. (Extended) Ionic compounds form a giant lattice of alternating positive and negative ions.
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, Give one, take one: ions.. When a metal meets a non-metal, electrons move from one atom to the other. 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: Losing and gaining electrons | Pick a metal and a non-metal in the simulator and move the outer electrons across. 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: Dot-and-cross diagrams | Draw one atom's electrons as dots and the other's as crosses, so the reader can see where each electron came from. | Pairs |
| 21 to 30 min | Teach and check: Giant ionic lattices | Extended. Project tab 3 of the lab (Chemical bonding lab, ionic lattice) and let a student drive it while the class predicts. | Connect-Extend-Challenge |
| 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 Giant ionic lattices, 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.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
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 |
|---|---|
| In calcium fluoride there are two separate F − ions, each with a single negative charge. | Do not draw an F 2 unit or write F 2 2−: the small 2 in CaF 2 counts ions, it does not mean the fluorine atoms are bonded together. |
| Molten or dissolved ionic compounds conduct because ions move and carry the charge. | Writing that 'electrons move' through molten sodium chloride scores no marks: free electrons are the explanation for metals and graphite, not for ionic compounds. |
Ions and ionic bonding: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by an ion and by an ionic bond.
Describe, in terms of electrons, how sodium chloride is formed from sodium atoms and chlorine atoms.
(Extended) Describe the dot-and-cross diagram for magnesium oxide. Include the configurations and charges of the ions formed.
(Extended) Calcium forms Ca2+ ions and fluorine forms F− ions. Deduce the formula of calcium fluoride and explain how the electrons are transferred.
Potassium bromide does not conduct electricity when solid but does conduct when molten or dissolved in water. Explain these observations.
(Extended) Explain why magnesium oxide has a very high melting point.
Answer key and marking notes
- Ion: a charged particle formed when an atom (or group of atoms) loses or gains electrons.
- Ionic bond: the strong electrostatic attraction between oppositely charged ions.
- Each sodium atom (2,8,1) loses one electron to form Na+ (2,8).
- Each chlorine atom (2,8,7) gains that electron to form Cl− (2,8,8).
- The oppositely charged ions attract each other strongly (ionic bonds).
- Magnesium (2,8,2) transfers two electrons to oxygen (2,6); they are shown as two crosses in the oxygen's outer shell.
- Magnesium ion: [2,8] with charge 2+.
- Oxide ion: [2,8] with charge 2−; each ion drawn in square brackets with its charge outside.
- Each calcium atom loses two electrons, one to each of two fluorine atoms.
- Two F− ions balance one Ca2+ ion, so the formula is CaF2.
- Potassium bromide is made of ions (K+ and Br−).
- In the solid the ions are held in fixed positions and cannot move.
- When molten or in solution the ions are free to move and carry charge.
- Magnesium oxide is a giant ionic lattice with strong electrostatic attractions between many oppositely charged ions.
- A large amount of energy is needed to overcome these attractions.
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 ion has the same electronic configuration as a neon atom (2,8)?
- ANa+
- BK+
- CCl−
- DLi+
The diagram shows a particle with 12 protons in its nucleus (12p) and all of its electrons. What is the particle?
- Aa magnesium ion, Mg2+
- Ba neon atom
- Ca magnesium atom
- Dan oxide ion, O2−
Element X is in Group VI of the Periodic Table. What is the charge on the ion formed when X reacts with a metal?
- A2−
- B6+
- C2+
- D6−
A white solid has a high melting point. It does not conduct electricity when solid, but it does conduct when dissolved in water. What type of substance is it most likely to be?
- Aan ionic compound
- Ba simple molecular substance
- Ca giant covalent substance
- Da metal
Aluminium (2,8,3) reacts with fluorine (2,7) to form aluminium fluoride. Which describes the electron transfer?
- AEach aluminium atom loses three electrons, one to each of three fluorine atoms, forming AlF3
- BEach aluminium atom gains three electrons, one from each of three fluorine atoms, forming AlF3
- CEach aluminium atom loses one electron to one fluorine atom, forming AlF
- DEach aluminium atom shares three pairs of electrons with three fluorine atoms
Which describes the structure of solid sodium chloride?
- Aa giant lattice of alternating Na+ and Cl− ions, held by strong electrostatic attractions in all directions
- Bseparate NaCl molecules held together by weak forces
- Cpairs of Na+ and Cl− ions, each ion attracted only to its partner
- Da lattice of sodium atoms and chlorine atoms sharing pairs of electrons
Magnesium oxide melts at 2852 °C but sodium chloride melts at 801 °C. Both are ionic. Which is the best explanation for the difference?
- AThe ions in magnesium oxide (Mg2+ and O2−) have larger charges, so the electrostatic attraction between them is stronger
- BMagnesium oxide has covalent bonds, which are stronger than ionic bonds
- CMagnesium oxide molecules are larger, so the intermolecular forces are stronger
- DMagnesium atoms have more electrons in their outer shell than sodium atoms, so they hold on to them more tightly
In a dot-and-cross diagram for sodium oxide, Na2O, oxygen's electrons are drawn as dots and sodium's as crosses. In the oxide ion, how many electrons are in the outer shell, and how many of them are crosses?
- A8 electrons, of which 2 are crosses
- B8 electrons, of which 1 is a cross
- C6 electrons, none of which are crosses
- D7 electrons, of which 1 is a cross
Quiz key
Sodium (2,8,1) loses one electron to form Na+, 2,8, the same as neon. K+ (from 2,8,8,1) is 2,8,8 and Cl− (from 2,8,7) is 2,8,8, both like argon. Li+ (from 2,1) is 2, like helium.
12 protons means the element is magnesium. There are only 2 + 8 = 10 electrons, two fewer than the protons, so the particle has a charge of +2: Mg2+. It looks like neon, but a neon atom has 10 protons. A magnesium atom would have 12 electrons (2,8,2), and an oxide ion has the configuration 2,8 but only 8 protons.
With six outer electrons, the atom gains two electrons to complete its outer shell of eight. Gaining two negative electrons gives a 2− ion (like O2− and S2−). Losing six electrons (6+) would need far too much energy; 2+ is the charge on Group II ions; 6− confuses the group number with the number of electrons gained.
Ionic compounds have high melting points because of the strong attraction between ions. In the solid the ions are held in fixed positions, so it does not conduct; when dissolved, the ions are free to move and carry charge. A simple molecular substance would have a low melting point. A giant covalent substance such as diamond does not conduct and does not dissolve. A metal conducts even when solid.
Aluminium loses its three outer electrons to become Al3+ (2,8). Each fluorine atom needs only one electron to become F− (2,8), so three fluorine atoms are needed: AlF3, and the charges balance (+3 + 3 × −1 = 0). The metal loses electrons rather than gaining them. Losing only one electron would leave aluminium without a full outer shell. A metal and a non-metal transfer electrons; sharing describes covalent bonding.
Sodium chloride is a giant ionic lattice: a regular 3D arrangement in which each Na+ is surrounded by Cl− ions and each Cl− by Na+ ions, with strong electrostatic attraction in every direction. There are no separate molecules or isolated ion pairs; the formula NaCl only gives the 1 : 1 ratio of ions. Shared electron pairs would be covalent bonding, not ionic.
Melting an ionic solid means overcoming the electrostatic attraction between the ions. Mg2+ and O2− carry double charges, while Na+ and Cl− carry single charges, so the attraction in magnesium oxide is much stronger and more energy is needed. Magnesium oxide is ionic, not covalent, and it contains no molecules, so intermolecular forces do not apply. In the compound, magnesium has already lost its outer electrons.
Oxygen (2,6) has six outer electrons (dots) and needs two more. Each sodium atom (2,8,1) gives one electron, and two sodium atoms react, so two crosses are added: the O2− ion has 8 outer electrons, 6 dots and 2 crosses. One cross would only give O− with 7 electrons, which is not a full shell. Six electrons is the oxygen atom before any transfer.
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.