On Memorising the Periodic Table

Oxford’s chemistry degree requires students to memorise the periodic table (not the mass numbers, but the positions of all the elements). What effect does this have on assessment validity? This blog explores how a series of representative exam questions are problematic in light of the memorisation requirement.

The Purpose of Higher Education

What makes Higher Education “higher”?

Some people appeal to the advanced content of study (perhaps the octet rule is lower-order education than MO theory). This perspective often draws the focus to specific concepts.

Some people appeal to the level of cognition (perhaps memorising “electronegativity is the ability of one atom to pull electron density towards itself in a covalent bond” is lower-order than interpreting the orbital coefficients of the MO diagram of HF). This perspective often draws the focus to specific verbs (to synthesise information is a higher order of learning than to comprehend it).

Some people appeal to a vaguer notion of Higher Education being about coming into a relationship with disciplinary knowledge. Perhaps you knew that water was v-shaped at school because of the two lone pairs, but now you think about the bending of water in terms of a Walsh diagram and the stabilisation of the HOMO through the s-p mixing permitted in the C2v point group. By this view there is perhaps a sophistication to a graduate chemist which a school-leaver develops during a degree.

In all three of these outlooks, memorisation is probably not the point of a University education: memorisation shouldn’t be too central in the work students need to do to get a degree (though there probably are some facts which it is convenient to memorise).

If memorisation isn’t really what we want students to do, then our assessments should reflect that, focusing instead on the ideas and arguments woven through advanced chemistry. I am going to analyse a few representative exam questions to try and explore what role memorisation of the periodic table plays in current assessments. How many marks can someone access at Oxford if they can do chemistry but can’t remember the periodic table?

Representative Exam Questions

VSEPR

Predict the structure of [SnCl5]-. [1]

To solve this problem, you need to count the number of electrons around tin and select the appropriate VSEPR shape. This gives you a trigonal bipyramidal (D3h) structure. This style of question is common in first year exams, and related problems often appear in point group and NMR problems at later stages.

The VSEPR analysis requires a correct electron count, and a correct electron count requires the student to recall that tin is in group 4. If students get the question right, they have demonstrated all of this, but what if they mis-remember? An incorrect count gives an incorrect answer.

Transition Metal Magnetism

Predict the magnetic moment of [CoCl4]2- [2]

To solve this problem you need to populate the tetrahedral splitting diagram with the correct number of electrons, and then apply the spin-only formula to convert the number of unpaired electrons into a magnetic moment.

If students get the right number at the end, they have demonstrated that they can do all of this. But what mark should a correct analysis of the wrong electron count attract? The student has shown they can do the substantive science - this is the higher-order thinking. Should their mark be docked because they mis-remembered which group cobalt is in? How big a penalty should recall activities incur here when the process knowledge is all in place?

Lanthanide Multiplet Symbols

State the ground state multiplet symbol (2S+1)L(J) of Ho3+. [1]

To get this question right, the student needs to extract the correct values of S and L from the configuration of 10 f-electrons associated with the ion. As 10 is larger than 7, J will be the sum of S and L (rather than the difference between them).

Is this a question about Hund’s Rules, or is it one about knowing where holmium is? At the moment, it’s both.

In fairness to recent examiners, the lanthanide sequence is sometimes given by doing something like labelling the x-axis of a graph. I still tell my students to learn the f-block elements because the examiners are entitled to withhold the sequence.

Organometallic electron Counting

Give the organometallic electron count of NbCp2(H)(Et) [1]

To solve this problem, a student needs to use either the ionic or the neutral scheme of counting to work out that the complex has 17 valence electrons. In either event, the factual recall of niobium being in group 5 is a necessary condition for getting the answer. A student can understand how to count electrons without remembering where niobium is; getting the mark bundles up both concepts.

Comparative Essay

Compare and contrast the chemistry of Ti, Sn, and Ce. [25]

To answer this descriptive question a student needs to be able to situate each of the elements within the themes and trends of the periodic table. This is hard to do if you don’t know where the elements are.

There’s lots to say here based on the fundamental atomic properties (4+ but for different reasons; identity of the next-most-stable oxidation state; extent of covalency in bonding etc), but making these chemical points requires students to know where all three elements are.

What mark does a student get if they make great chemical arguments but mis-remember Sb for Sn (‘it begins with an S’) or Ce for Eu (‘it’s one of those weird lanthanides’)? It’s not answering the question if the element is the wrong one - presumably the penalty will be large if the exam is to be rigorous.

Authenticity 

Every time I have needed a periodic table as a ‘real’ scientist, I have had one. It’s in the back of every textbook, it’s on posters in offices, it’s taped up in the lab above the balance. My current job - teaching inorganic chemistry at Oxford - is literally the thing my degree prepared me best for, and I still don’t know the f-block.

Memorising the periodic table is not something students need to become great chemists: it’s something they need to succeed at the specific task of doing well in Oxford’s chemistry exams. But exams are incredibly high-stress situations, and silly mistakes are easy to make when stressed.

More pressingly, time spent learning the periodic table comes at the cost of time spent doing something else. Every year examiners comment on some concept or fact student hadn’t quite got - how many of them would have understood dynamic Jahn-Teller distortions or the one-quarter shell effect or the entactic state but for the time they spent really nailing where Re Rh and Ru were?

Conclusion: Give Them The Periodic Table!

If higher education is about higher-order learning, then our exams are doing the wrong thing when they demand students to recall a fact they will be able to access at a glance in professional settings: we’re just not measuring how good students are at chemistry when we examine like this.

Oxford could improve the validity of its inorganic assessments by providing students with a periodic table. At the moment we are often examining someone’s recall rather than their reasoning. Providing students with the periodic table would improve the rigour of our assessments because we could always be confident that we were testing students’ thinking rather than their memory.