“You have not learnt anything in Chemistry if you don’t know about the exceptional cases”
These words of my Chemistry teacher still ring true in my ears as I reminisce the JEE Advanced preparations days. More often than not, and surely not to students’ liking, it is these exceptions that exams like JEE Advanced exploit and base their questions on. In that scenario, welcome to the mother of all exceptions in Chemistry (at least the one you’re concerned for JEE preparation) : Lanthanide Contraction.
Before I move on to the more exam oriented stuff regarding lanthanide contraction, let us first get on to the technical aspects of what lanthanide contraction is in the first place. Here’s a kinda bookish definition of lanthanide contraction:
Book Definition :
‘Lanthanide contraction is a term used in chemistry to describe the greater-than-expected decrease in ionic radii of the elements in the lanthanide series from atomic number 57, lanthanum (La), to 71, lutetium (Lu).’
This utterly imaginative term was coined by the Norwegian geochemist Victor Goldschmidt in his series “Geochemische Verteilungsgesetz der Elemente”. I shall advise the reader here to ditch trying to pronounce the treatise name. You’ll have an easier time solving questions on lanthanide contraction than pronouncing that. Believe me, I’ve done both 😛
Much as the definition may sound Greek (Nordic, maybe?) to you at first glance, the principle behind it is simpler than one can imagine. So, let’s first analyse what is this almighty thing that lanthanide contraction is an exception to? *Puts on a thinking face*
Answer: Technically, nothing!
Well, sorry to be the party-pooper, but the actual phenomenon of lanthanide contraction is no exception to any normal periodic property. However, it is the consequence of lanthanide contraction that happens to lead to an exception.
Lanthanide Contraction : In Detail
As the ionic radii of the elements in the lanthanide series keeps on decreasing more than we normally expect (this is not an exception, because ionic radii does decrease along a period, albeit in a less rapid fashion), it’s the elements just after the lanthanide series which bear the mark of being exceptional cases.
Because of lanthanide contraction, the elements just after the entire lanthanide series actually have ionic radii less than that of their predecessors in the same group. This ‘decrease in the size’ is the mother of exceptions that I had proclaimed of so zealously.
Much as this seems like a passing property of the lanthanide series, do not be deceived. For this very thing shall haunt the daylights out of you till you till you are done with the second paper of JEE Advanced. Even the most innocuous question in JEE Advanced can turn into a death trap because it has been laced with lanthanide contraction. It simply is because lanthanide contraction has got a mammoth’s footprint in inorganic chemistry. Before long, you’ll be spouting ‘lanthanide contraction’ to solve every problem which can’t be solved in less than 180 seconds (don’t do that).
Let me give you an example of how widespread lanthanide contraction (or it’s subsequent immediate effect) is. Consider this bit of information:
The atomic radius of the metal zirconium, Zr, (a period-5 transition element) is 159 pm and that of hafnium, Hf, (the corresponding period-6 element) is 156 pm. The ionic radius of Zr4+ is 79 pm and that of Hf4+ is 78 pm. The radii are very similar even though the number of electrons increases from 40 to 72 and the atomic mass increases from 91.22 to 178.49 g/mol.
Zirconium and hafnium therefore have very similar chemical behaviour, having closely similar radii and electron configurations. Radius-dependent properties such as lattice energies, solvation energies, and stability constants of complexes are also similar. Because of this similarity hafnium is found only in association with zirconium. Titanium, on the other hand, is in the same group but differs enough from those two metals that it is seldom found with them.
Thus, lanthanide contraction leads to formation of pairs of elements, from the same group, which have very similar properties between them. These elements are known as chemical twins. Zr-Hf is one example.
Importance from JEE point of view
Apart from chemical twins, we also see a lot of importance to lanthanide contraction in coordination chemistry. It also plays a major role (at least in formation of typical JEE type tricky questions) in general d-block and f-block inorganic chemistry. Given the fact that inorganic chemistry forms about ⅓ rd of the weightage (if recent trends are to be followed, the weightage is inching towards 40% even) in JEE, there is ample of space for introduction of lanthanide contraction related questions. Watch out for about two in the papers.
That is all I could possibly tell you about lanthanide contraction. It seems like a side topic, true, but it indeed knows how to punch above its own weight effectively. It is the small things in JEE that make the biggest difference. C’est la vie gens (I don’t trust my French, I hope that means : This is life people). If you are aiming for a decent enough rank in JEE, best brush up on your knowledge of lanthanide contraction.
Hope this was helpful and now you have a complete understanding of this exceptional phenomena. From the exam point of view, indeed, inorganic chemistry is a important part, but physical and organic chemistry are equally important too. Here are some tips and tricks to become a pro at physical chemistry as well as master in organic chemistry.
All the best 🙂