Year 8 Science · Unit 2 · Lesson 9
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Learning Goals
Match each ion to its particles
Write the matching letter in the middle column. Each letter is used once.
| Ion | Letter | Protons and electrons |
|---|---|---|
| Na⁺ | A. 17 protons, 18 electrons | |
| Cl⁻ | B. 13 protons, 10 electrons | |
| Mg²⁺ | C. 9 protons, 10 electrons | |
| O²⁻ | D. 11 protons, 10 electrons | |
| Al³⁺ | E. 12 protons, 10 electrons | |
| F⁻ | F. 8 protons, 10 electrons |
Put the salt-making steps in order
Number the steps 1 (first) to 4 (last) for sodium and chlorine atoms becoming table salt.
| Step | What happens |
|---|---|
| Sodium now has 11 protons, 10 electrons (Na⁺); chlorine has 17 protons, 18 electrons (Cl⁻). | |
| A sodium atom (2, 8, 1) and a chlorine atom (2, 8, 7) meet during a reaction. | |
| The positive and negative ions attract and pack together as sodium chloride. | |
| Sodium’s single outer electron moves across to the chlorine atom. |
1. Which three ions in the match table formed by losing electrons? What sign is their charge, and what is this kind of ion called?
2. Potassium (2, 8, 8, 1) reacts with fluorine (2, 7) in the same way. Write the ion each atom forms, say if it is a cation or anion, and give its new electron arrangement.
| Ion | Symbol | Mass in 1 kg of seawater (g) |
|---|---|---|
| Chloride | Cl⁻ | 19.4 |
| Sodium | Na⁺ | 10.8 |
| Magnesium | Mg²⁺ | 1.3 |
| Calcium | Ca²⁺ | 0.4 |
| Potassium | K⁺ | 0.4 |
Data: approximate values for typical seawater, rounded. A few other ions are not shown.
3. Add up the mass of the cations in Table 1 and compare it with the anion.
Cations g
A student claims
“There is more anion mass than cation mass, so seawater must be negatively charged.”
4. Seawater has no overall charge. Give two reasons the claim is wrong. Think about the mass of each ion.
5. For lithium (2, 1), sulfur (2, 8, 6) and argon (2, 8, 8), predict whether each forms a cation, an anion or no ion, and write the symbol of any ion. Explain each prediction using a full outer shell, then explain why the sulfur ion is still sulfur.
Match each ion to its particles
Na⁺ → D; Cl⁻ → A; Mg²⁺ → E; O²⁻ → F; Al³⁺ → B; F⁻ → C.
1. Na⁺, Mg²⁺ and Al³⁺. Losing electrons leaves more protons than electrons, so the charge is positive; these are cations. [2 marks: 1 three ions, 1 positive and named cations]
Put the salt-making steps in order
Reading down the table as printed: 3, 1, 4, 2.
2. Potassium loses its 1 outer electron and forms a cation, K⁺, arrangement 2, 8, 8. Fluorine gains that 1 electron and forms an anion, F⁻, arrangement 2, 8. Both now have a full outer shell. [3 marks: 1 each ion with symbol, 1 both arrangements]
Go Deeper
3. 10.8 + 1.3 + 0.4 + 0.4 = 12.9 g of cations, compared with 19.4 g of chloride, so the anion mass is 6.5 g greater. [2 marks: 1 total, 1 comparison]
4. Any two, explained: (i) charge depends on the number of ions and the charge on each, not on their mass; (ii) each chloride ion is heavier than each sodium ion (about 35 compared with 23), so equal numbers of Na⁺ and Cl⁻ give more chloride mass with the charges still balanced; (iii) Mg²⁺ and Ca²⁺ each carry two positive charges, so they balance more negative charge per ion; (iv) the table leaves out some ions, so it cannot show the full charge balance. [3 marks: 1 each reason, 1 clear link to charge]
Show What You Know
5. Marking guide (points expected): 1 lithium forms a cation, Li⁺; 1 it has 1 outer electron, and losing 1 is easier than gaining 7, leaving a full shell (2); 1 sulfur forms an anion, S²⁻; 1 it has 6 outer electrons, and gaining 2 is easier than losing 6, giving 2, 8, 8; 1 argon forms no ion because its outer shell is already full (2, 8, 8); 1 the sulfur ion still has 16 protons, and the number of protons decides the element. [6 marks]
Common error to watch for: in the match, choosing the letter from the charge alone. Five of the six ions have 10 electrons, so students must use the proton number to pick the element.