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Writing good answers to AP Periodicity problems November 15, 2012 by Adrian
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In my experience, the majority of kids understand most of the chemistry behind the periodic trends typically asked in AP, free response questions, but way too often they cannot express that knowledge in succinct, meaningful English that allows (and encourages) the graders to award credit. As a result, students tend to score poorly on a topic that really ought to be an area that yields more points. Below I have attempted to summarize some of the problems of expression (not necessarily issues with understanding the chemistry) to hopefully help to improve those answers and scores.
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1. There is something that sits very poorly with me about the following sentence (and others like it).
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“Sodium and magnesium are in the same shell’. Sodium and magnesium are in the same period, and their outer electrons are in the same quantum level (shell) – there is a subtle, but important difference! I suggest applying ‘quantum level’ only to electrons, and ‘period’ to elements. 2. The use of term ‘shielding’ to describe the relative numbers of filled quantum levels between the outer electrons and the nucleus is absolutely fine, but I much prefer ‘shielding’ to be used exclusively in the context of describing ionization energy trends, and NOT when describing size trends. When describing size trends, IMO it is better to refer to the number of ‘occupied quantum levels’ causing differences in size. Of course, ‘occupied quantum levels’ are directly related to shielding, but nevertheless.
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3. A classic mistake is to re-state the question, and this can apply to many topic areas other than just periodicity, but it seems to be particularly popular, here. If the question states that a particular trend is in play, e.g., ‘The first ionization energy of neon is greater than that of lithium’, then I have no interest in answers that start with sentences like, ‘As you go across a period the first ionization energy increases’. We already KNOW that, the question told us! That habit inevitably leads us to a classic error in periodicity answers, i.e.,…. …..4. Never state the ‘trend’ as the reason. For example, it is not acceptable to state that the reason that neon has a higher first ionization than lithium, is that neon is further to the right in the period, and that ionization energy increase left to right on the table; (even though the second statement here, is true). 5. Answers that suggest it is harder to remove electrons from full, ‘stable’ shells, are not my favorite thing, either.
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For example, the reason Ca’s 3rd ionization energy is so much higher than its first and second, is not that Ca2+ is ‘stable’ and has a ‘noble gas configuration’. Rather it is because the third electron is being removed from 3p sub-level (as opposed to 4s sub-level), which is closer to the nucleus and the shielding is less making the attraction stronger.
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6. I am a huge fan of writing the electronic configurations of any species involved in ionization energy difference questions. This very often partially answers the question anyway, but in any case it can make it much easier for the student to ‘see’ the answer.
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For example, if we actually write the electronic configurations for Ne and Na as part of the answer
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when explaining the differences in their first ionization energy (Ne: 1s2 2s2 2p6 and Na: 1s2 2s2 2p6 3s1), then there is immediately an obvious difference in the quantum shell where that first electron resides. The written electronic configurations act like ‘diagrams’, in as much as they help to explain and ‘illustrate’ what many, garbled words may fail to do. For that reason, I say write electronic configurations when comparing ionization energies. Oh, and one more thing. Make sure you are looking at the Periodic Table that comes with the exam when you are answering these questions!
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