Year 8 Science · Unit 2 · Lesson 07
Challenge Worksheet
Learning Goals
Find the errors
This explanation contains exactly three errors. Underline each one and write the corrections below.
A student’s explanation of ³²₁₆S
The bottom number, 16, is the atomic number, so the atom has 16 protons. The top number is the mass number, so the atom has 32 neutrons. As the atom is neutral, it has 32 electrons. Adding one neutron would give sulfur-33, still sulfur. Taking away one proton would still leave sulfur, because only the mass number changes.
1. Write the three corrections.
2. A news report says: “At ANSTO, scientists make technetium-99m (Z = 43, A = 99) for medical scans. Its mass number is 99, which means each atom has 99 protons.” Identify the error in the report. State what Z = 43 and A = 99 really tell us, and give the number of protons, neutrons and electrons in a neutral technetium-99m atom.
| Atom | Z | A | Neutrons | Neutrons ÷ protons (2 decimal places) |
|---|---|---|---|---|
| Helium-4 | 2 | 4 | 2 | 1.00 |
| Iron-56 | 26 | 56 | ||
| Copper-63 | 29 | 63 | ||
| Gold-197 | 79 | 197 | ||
| Uranium-238 | 92 | 238 |
Data: atomic and mass numbers of real atoms, from the periodic table.
3. Complete the last two columns of Table 1. Show your working for uranium-238.
Uranium-238: neutrons = , neutrons ÷ protons =
4. Use your completed Table 1 to describe how the number of neutrons compares with the number of protons as atoms get heavier. Quote two values.
5. Evaluate this claim: “An atom with a bigger mass number always has more protons.” Test it using potassium-39 (³⁹₁₉K) and argon-40 (⁴⁰₁₈Ar), and explain what your result shows about the two numbers.
6. Evaluate: “Only the atomic number really matters; the mass number is just extra detail.” Predict what happens to carbon-12 (Z = 6, A = 12) if one proton is added, and if instead one neutron is added. Use one other example from this sheet, and finish with a clear judgement.
Find the errors
1. (i) The atom has 32 − 16 = 16 neutrons, not 32; 32 is protons plus neutrons. (ii) A neutral atom has 16 electrons, the same as its protons, not 32. (iii) Taking away a proton changes the element: with 15 protons it would be phosphorus, not sulfur. [3 marks: 1 per correction]
The news report
2. The report treats the mass number as the proton count. Z = 43 is the number of protons (which makes it technetium); A = 99 is protons plus neutrons. So 43 protons, 99 − 43 = 56 neutrons, 43 electrons. [3 marks: 1 error; 1 meaning of Z and A; 1 all three counts]
Your Turn
3. Iron-56: 30, 1.15. Copper-63: 34, 1.17. Gold-197: 118, 1.49. Uranium-238: 238 − 92 = 146; 146 ÷ 92 = 1.59. [3 marks: 1 neutrons column; 1 ratio column; 1 uranium working]
4. Light atoms have about the same number of neutrons as protons (helium-4, 1.00), but heavier atoms have more neutrons than protons, and the gap grows (iron-56, 1.15; uranium-238, 1.59). [2 marks: 1 pattern, 1 two values quoted]
Go Deeper
5. The claim is false. Argon-40 has the bigger mass number (40) but only 18 protons; potassium-39 has a smaller mass number (39) but 19 protons. Argon has more neutrons (22 compared with 20). This shows the mass number includes neutrons, so it cannot be used to rank or identify atoms by protons; only Z gives the proton count. [3 marks: 1 verdict, 1 correct counts, 1 conclusion about A and Z]
Show What You Know
6. Marking guide (points expected): 1 adding a proton gives Z = 7, A = 13, so the atom becomes nitrogen; 1 adding a neutron gives Z = 6, A = 13, so it is still carbon; 1 the atomic number (proton count) decides the element; 1 the mass number tells you how many neutrons there are and how heavy the nucleus is, which Z cannot tell you (for example carbon-12 and carbon-13, or argon-40 compared with potassium-39); 1 another example from the sheet used correctly (for example technetium-99m or uranium-238); 1 a supported judgement: the statement is only partly right, because Z alone identifies the element but you need A as well to describe the particular atom. [6 marks]
Common error to watch for: in Q3, dividing protons by neutrons instead of neutrons by protons. Every ratio in the column should be 1.00 or more.