WJEC Chemistry for AS Level: Revision Workbook

Question and mock answer analysis Q & A 1 Tellurium is one of the rarest stable elements present on Earth. It often forms tellurides containing Te 2− ions. (a) Give the number of protons and electrons present in a telluride anion. [1] (b) Tellurium forms a range of radioactive isotopes including 107 Te, 117 Te and 127 Te. (i) The isotope 107 Te can decay by emission of an Ƚ -particle or a positron. Identify the isotopes produced from 107 Te by these two decay methods. [2] (ii) The isotope 127 Te releases a Ⱦ -particle when it decays. State what is meant by a Ⱦ -particle. [1] (iii) The half-life of the isotope 117 Te is 62 minutes. A sample of 80 mg of the isotope decays to leave 5 mg of the isotope after t minutes. Calculate the value of t . [2] (c) Explain why radioactivity is harmful to health. [2] Interpretation In this unit, you build upon your GCSE work, and are expected to recall your work on radioactive decay and atomic structure. In this question you should apply your knowledge of the formation of ions and radioactive decay to find the numbers of protons, neutrons and electrons in various species – it is important to remember that if the number of protons changes then the element symbol must also change. You are only expected to use full half-lives in your calculations. Remember time can be given in a range of units and the answer must match. Dewi’s answer (a) 52 protons and 54 electrons ض (b) (i) Alpha decay produces 103 Sn and positron emission produces 107 I غض (ii) -particles are fast-moving electrons. ض (iii) 80 mg ÷ 5 mg = 16 = 2 4 so four half-lives Time t = 62 × 4 = 248 minutes ضض (c) Radioactive particles are high-energy and can damage biological molecules. In large doses this can denature enough molecules to cause death, but smaller doses can damage DNA leading to mutations and then cancerous tumours. ضض MARKER COMMENTARY (a) The correct numbers of protons and electrons. (b) (i) The mass numbers in both cases are correct; however, Dewi has increased the atomic number by 1 when the positron is emitted rather than decreased it by 1 and so does not gain the second mark. (ii) The definition here is correct and gains 1 mark. (iii) This method is correct and gains both marks. (c) This answer is complete and gains 2 marks. Overall Dewi gains 7 out of 8 marks as he recalls the factual content in detail and can apply ideas to find the numbers of protons, neutrons and electrons in many species. 1.2: Basic ideas about atoms | 25 Unit 1 Practice questions