Year 8 Science · Unit 2 · Lesson 9
Challenge Worksheet
Learning Goals
Find the errors
This homework answer contains exactly three errors. Underline each one and write the correction below.
A student’s homework
A magnesium atom (2, 8, 2) gains 6 electrons to fill its outer shell, so it forms the ion Mg⁶⁻. An oxygen atom (2, 6) gains 2 electrons, so the oxygen ion now has 10 protons. A chlorine atom gains 1 electron and becomes Cl⁻, which is an anion. A potassium atom loses 1 electron, so K⁺ has 18 electrons, the same as argon, which means it has turned into argon.
1. Write the three corrections.
Identify the mystery ions
Use the number of protons to name the element. Then write the symbol of the ion, with its charge.
| Ion | Protons | Neutrons | Electrons | Element | Symbol and charge |
|---|---|---|---|---|---|
| a | 12 | 12 | 10 | ||
| b | 17 | 18 | 18 | ||
| c | 19 | 20 | 18 | ||
| d | 9 | 10 | 10 |
2. Work backwards. An ion has a charge of 2−, 18 electrons and a mass number of 32. Find its number of protons, name the element, and find its number of neutrons.
Element neutrons:
3. Ion b and ion c in the table have exactly the same number of electrons. Explain why they still have opposite charges, and why neither one has become argon.
Evaluate the claim
Someone claims
“Sodium is a metal that reacts violently with water, and chlorine is a poisonous gas. Table salt is sodium chloride, so it must be dangerous to eat.”
| Substance | Particles it is made of | Melting point (°C) | Safe to eat? |
|---|---|---|---|
| Sodium | sodium atoms | 97.8 | No: reacts violently with water |
| Chlorine | Cl₂ molecules | −101.5 | No: poisonous gas |
| Sodium chloride | Na⁺ and Cl⁻ ions | 801 | Yes, in normal amounts |
Data: standard values for the pure substances.
4. Use Table 1 to give the part of the claim that is correct, and two ways sodium chloride differs from both elements.
5. Evaluate the claim. Compare a sodium atom (2, 8, 1) with a sodium ion, and a chlorine atom (2, 8, 7) with a chloride ion, and use the idea of a full outer shell to explain why salt behaves so differently from the elements.
Find the errors
1. (i) Magnesium loses its 2 outer electrons (losing 2 is easier than gaining 6), forming Mg²⁺. (ii) The oxygen ion still has 8 protons; it has 10 electrons. (iii) K⁺ is still potassium: it keeps its 19 protons; having 18 electrons does not change the element. The chlorine sentence is correct. [3 marks: 1 per correction]
Identify the mystery ions
a: magnesium, Mg²⁺. b: chlorine, Cl⁻. c: potassium, K⁺. d: fluorine, F⁻.
2. Charge 2− means 2 more electrons than protons: 18 − 2 = 16 protons, so the element is sulfur (S²⁻). Neutrons = 32 − 16 = 16. [3 marks: 1 protons, 1 sulfur, 1 neutrons]
3. Ion b has 17 protons and 18 electrons, so it is −1 (it gained an electron); ion c has 19 protons and 18 electrons, so it is +1 (it lost an electron). The element is decided by protons, not electrons: argon has 18 protons, so neither ion is argon (b is still chlorine, c is still potassium). [3 marks: 1 each charge with reason, 1 protons decide element]
Evaluate the claim
4. Correct part: sodium metal and chlorine gas are both dangerous (sodium reacts violently with water, chlorine is poisonous). Different properties (any two): sodium chloride is made of ions, not atoms or molecules; its melting point (801 °C) is far higher than either element; it is safe to eat in normal amounts. [3 marks: 1 correct part, 1 each property]
Show What You Know
5. Marking guide (points expected): 1 true part: sodium metal and chlorine gas are dangerous; 1 flaw: salt does not contain sodium atoms or chlorine molecules, it contains Na⁺ and Cl⁻ ions; 1 the sodium atom (2, 8, 1) loses 1 electron to become Na⁺ (2, 8), charge +1; 1 the chlorine atom (2, 8, 7) gains 1 electron to become Cl⁻ (2, 8, 8), charge −1; 1 both ions have a full outer shell, a stable arrangement, so they do not react like the elements; 1 judgement with Table 1 evidence: the claim is wrong, salt has very different properties and is safe in normal amounts. [6 marks]
Common error to watch for: saying the ions are safe because they are “a different element”. They are still sodium and chlorine (same protons); only the number of electrons, and so the behaviour, has changed.