Year 8 Science · Unit 2 · Lesson 22
Challenge Worksheet
Learning Goals
Find the errors
This summary has exactly three errors. Underline them and correct them below.
A student’s summary of sodium
Sodium (Na) has atomic number 11, so every sodium atom has 11 protons. The common isotope sodium-23 has 11 protons and 11 neutrons. Sodium is in Group 1, so, like potassium, it is a very reactive metal. When a sodium atom loses one electron it becomes the ion Na⁻. Table salt, sodium chloride, contains sodium, so salt also reacts strongly with water.
1. Write the three corrections.
From particles to the periodic table
| Ion | Charge | Electrons | Mass number | Protons | Neutrons | Element | Group |
|---|---|---|---|---|---|---|---|
| P | 3+ | 10 | 27 | ||||
| Q | 2− | 10 | 16 | ||||
| R | 2+ | 18 | 40 |
Hint: a 3+ ion has lost three electrons; a 2− ion has gained two. Use a periodic table for the element and group.
2. Complete the four empty columns of Table 1.
3. Kai says: “P and Q both have 10 electrons, so they must be the same element.” Evaluate Kai’s claim. Then use the groups of P and Q to explain why one forms a positive ion and the other a negative ion.
Scenario: the unlabelled grey block
Priya finds a small grey metal block in the prep room. Its label has worn off. The technician says it is one of the four metals in Table 2.
| Metal | Atomic number | Group | Density (g/cm³) | Melting point (°C) |
|---|---|---|---|---|
| Nickel (Ni) | 28 | 10 | 8.91 | 1455 |
| Zinc (Zn) | 30 | 12 | 7.14 | 420 |
| Tin (Sn) | 50 | 14 | 7.29 | 232 |
| Lead (Pb) | 82 | 14 | 11.34 | 327 |
Data: values for the pure metals, rounded (tin in its common white form).
Plan an investigation
Plan how Priya can find the density of the block. She has a balance and a measuring cylinder of water.
What she will measure, and with which equipment
Method
4. Priya’s results: the block has a mass of 71.0 g, and the water level rises from 40.0 mL to 50.0 mL when it is lowered in. Calculate the density of the block (1 mL = 1 cm³) and name the metal that matches best.
Density = g/cm³ Best match:
5. The measuring cylinder can only be read to the nearest 0.5 mL, so the volume could really be anywhere from 9.5 to 10.5 cm³. Explain why Priya cannot yet be sure which metal she has. Suggest one further test, using Table 2, that would settle it.
6. Evaluate this claim: “Chemistry is just a list of facts to memorise. Knowing where an element sits in the periodic table cannot help you work out anything new.” Use at least two examples from this unit, and finish with a clear judgement.
Find the errors
1. (i) Sodium-23 has 12 neutrons, not 11 (23 − 11 = 12). (ii) Losing one electron gives Na⁺ (a 1+ ion), not Na⁻. (iii) Sodium chloride is a new substance; it does not react with water like sodium metal (it dissolves). [3 marks: 1 per correction]
From particles to the periodic table
2. P: 13 protons (10 + 3), 14 neutrons (27 − 13), aluminium (Al³⁺), Group 13. Q: 8 protons (10 − 2), 8 neutrons, oxygen (O²⁻), Group 16. R: 20 protons (18 + 2), 20 neutrons, calcium (Ca²⁺), Group 2. [3 marks: 1 per correct row]
3. Kai is wrong: protons, not electrons, decide the element. P has 13 (aluminium), Q has 8 (oxygen). Aluminium is a Group 13 metal, and metals lose electrons (positive ions); oxygen is a Group 16 non-metal, and non-metals gain electrons (negative ions). [3 marks: 1 protons decide, 1 P and Q protons, 1 lose/gain]
Plan an investigation
Measure mass (balance) and volume by water displacement (cylinder). Method: weigh the dry block; read the water level, lower the block in until covered, read again; volume = difference; repeat and average.
4. Volume = 50.0 − 40.0 = 10.0 cm³. Density = 71.0 ÷ 10.0 = 7.10 g/cm³. Best match: zinc (7.14 g/cm³). [3 marks: 1 volume, 1 density, 1 zinc]
5. Density could be 6.8 (71.0 ÷ 10.5) to 7.5 g/cm³ (71.0 ÷ 9.5); zinc (7.14) and tin (7.29) both fit, so density cannot separate them (nickel and lead are ruled out). Test: heat a small piece; tin melts at 232 °C, zinc not until 420 °C. Also accept a larger sample or narrower cylinder. [3 marks: 1 range, 1 both zinc and tin fit, 1 valid test]
Show What You Know
6. Marking guide (points expected): 1 the claim is mostly wrong; 2 (1 each) two examples, such as Mendeleev (1869) leaving gaps and predicting undiscovered elements; the group predicting ion charge (Group 2 calcium forms 2+ ions, Group 16 oxygen forms 2− ions, Table 1); reactivity increasing down Group 1; 1 the table works because it is ordered by atomic number, so position carries meaning; 1 limitation: it gives patterns, not every value (Q4 and Q5 still needed measured data); 1 judgement backed by the examples, for example “Facts matter, but the table connects them so we can predict and explain.” [6 marks]
Common error to watch for: in Q2, taking protons from the mass number (27 for P) or applying the charge the wrong way. A positive ion has more protons than electrons; a negative ion has fewer.