WJEC Chemistry for AS Level: Revision Workbook

Rebecca’s answer (a) 52 protons and 52 electrons غ (b) (i) The radioactivity produces tin and the positron produces antimony. غغ (ii) -particles are electrons. غ (iii) 80mg — 40mg — 20mg — 10mg — 5mg Four half-lives so the time is 62 × 4 = 248 minutes ضض (c) Radioactivity can break bonds in DNA and cause mutations. This can then lead to the development of cancers. ضض MARKER COMMENTARY (a) Rebecca writes the same number of protons and electrons in an ion which is a common error. (b) (i) The question asks for the isotopes produced, but no mass numbers are given so it is awarded 0 marks. (ii) Although Ⱦ -particles are electrons there is no reference to their speed or energy, so no marks are awarded. (iii) This is the most common correct method seen for calculations using half-lives and gains 2 marks. (c) This is sufficient for both marks as she discusses damage to DNA and its effect. Overall Rebecca gains 4 out of 8 marks. She has not read the question carefully enough, leading to simple errors such as giving an answer for an atom in place of an ion and identifying an element and not a specific isotope. Q & A 2 This question refers only to the elements in the section of the periodic table shown: Li Be B C N O Na Mg Al Si P S (a) Use arrows in boxes to show the electron arrangement for nitrogen. [1] (b) Identify the element with the lowest first ionisation energy. Give reasons for your answer. [2] (c) State which of the elements Be and B will have the greatest ionisation energy. Explain your answer. [2] (d) Magnesium usually forms Mg 2+ in its compounds: Mg (g) r Mg 2+ (g) +2 e − The energy change for this process is 2187 kJ mol −1 . Suggest a value for the second ionisation energy of magnesium, giving a reason for your answer. [2] Interpretation These questions include some which require explanations and some which ask for reasons. ‘Explain’ questions require an answer which refers to the basic principles or ideas, in this case the factors which govern the strength of the attraction between nucleus and electron. Questions requiring reasons do not need the same depth of answer, and in this case discussion of patterns in ionisation energies is enough. 26 | WJEC Chemistry for AS Level: Revision Workbook | Unit 1: The Language of Chemistry, Structure of Matter and Simple Reactions