A problem with scandium
Tuesday 16 December 2014
(Note: This blog was first written in February 2013 and referred to the 2007 programme (for last exams in 2015). See the update added at the end which concerns the new (2014) programme.)
The IUPAC Compendium of Chemical Terminology (called The Gold book after its initial main author, Victor Gold) contains the following internationally accepted definition of a transition metal, “An element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.”(It is free to download this book from the Internet and I highly recommend that you do as it is a useful addition to a chemistry teacher’s resources).
This definition of a transition element is completely at odds with the current (2007) IB Higher Level Chemistry syllabus. Assessment statement 13.2.2 states “Explain why Sc and Zn are not considered to be transition elements.” The electronic configuration of scandium is [Ar]4s23d1. Scandium (see image on right) is therefore an element whose atom has an incomplete sub-shell and so according to the IUPAC definition it is clearly a transition element.
Often in chemistry, particularly at school level, definitions get simplified and sometimes these simplifications can cause problems. The IB has decided that scandium should not be considered a transition element because its only common ion is the Sc3+ ion which has an empty d sub-level (in fact scandium shows an oxidation state of +2 in the blue-black compound CsScCl3). The worrying factor about this simplification is that students who use the correct definition and go against what is written in the syllabus can be penalised. In fact the syllabus does not ask students to define a transition element. Where it does ask for specific definitions the IB is quite responsive to the fact that definitions may vary globally. Examiners and standardization teams are asked to take this into account when setting papers and fine-tuning markschemes. This, of course, is especially important for authors and examiners who are involved in different examination boards.
The problem comes when the IB itself has used the wrong definition. Consider a question asked on one of the Higher Level May 2010 exams. The question was worth three marks and asked, “ Explain why copper is considered a transition metal while scandium is not”. Clearly a difficult question to answer correctly as the initial statement is false. A good student should get credit if he or she challenges this statement but under the current rules only answers that agree with the syllabus statement are accepted. Hopefully, when the new curriculum appears for first teaching in 2014, chemistry based on mistaken definitions will have been eradicated.
UPDATE 16 December 2014
The IB has corrected this in the new programme (for first exams in 2016). In Topic 13.1 : First-row d-block elements it just states that 'zinc is not considered to be a transition element' and therefore by implication the IB does now consider scandium to be a transition element.