By Ron B. Davis Jr., Georgetown University
A curious property of the f-block is that similar elements are grouped horizontally, rather than by columns. The entire sixth row of f-block elements take the name lanthanides or lanthanoids because they are all very similar. The seventh-row f-block elements, all called actinides or actinoids, have their own horizontal similarities. So why the change in organization? What is the compelling reason behind this classification? Read on to find out.
Horizontal Grouping in D-block and F-block
Grouping f-block elements horizontally, by similarities that they share, produces groups of just two elements each. Another reason to change the organization of the f-block is that the horizontal groupings—just like vertical groupings and diagonal groupings—often bring similar elements together.
One can see a smaller version of horizontal grouping even in the d-block. It is important to remember that d-electrons, in transition metals, are held physically closer to the nucleus than valence electrons. The fact that the changing d-shell is deeper inside the atoms of the transition metals than their valence electrons, helps us understand the iron triad of iron-cobalt-nickel, and other horizontal similarities observed among d-block elements.
Thus, a similar kind of deep layering is observed here, as the f-subshell electrons are buried not one, but two principal shells, deep within the electron cloud.
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The Lanthanides Series
This results in very strong similarities among elements in the lanthanide series which have a 6s2 subshell in the outermost layer. Praseodymium is a 6s2-4f4 element, and promethium is a 6s2-4f5. Yet, because the 6s2 subshell comprises the outermost face of the electron cloud, these elements have many remarkably similar properties such as melting points and densities. They even produce similar ions, with nearly all of them favoring plus 3 oxidation states.
The lanthanides all behave a lot like lanthanum, even though lanthanum has no f-shell electrons, because they all have the same outer shell configuration as lanthanum.
One consequence of having identical electrons in their outermost layers and very similar atomic radii is that the lanthanide elements are very often found mixed together in nature. Not only that, the elements scandium and yttrium often come along for the ride.
Ytterbite, the ‘New Earth’ Element
In 1787, in a small quarry in Ytterby, Sweden, a strange black mineral, named ytterbite, was discovered. A Finnish chemist, by the name of Yohan Gadolin, isolated a substance from that mineral that he called ‘new earth’.
The alchemists’ idea of earth-air-fire-and-water had not been entirely abandoned at the time, and the term ‘earth’ was commonly used to refer to anything that could not be purified further. In that mineral, were discovered oxides of the elements scandium and yttrium.
And that wasn’t all that the minerals from Ytterby had to share. Yttrium oxide yielded oxides of terbium and erbium—all named for the same village.
Erbium and terbium were so hard to tell apart that their names switched around 1860. Later, erbium samples were found to contain ytterbium. Thus, four very similar elements were all named for the village of Ytterby. It didn’t just end there. In fact, even more hidden elements were discovered together with erbium. So, the next names went broader. Holmium was named for Stock-holm and thulium got its name from ‘Thule’, an old name for Sweden.
Cerium
Meanwhile, back in 1803, a second line of discoveries started with cerium, which is actually the most common member of the entire group, and more common than copper. Cerium is so common that it was originally discovered outside the Ytterby minerals. Its name is also entirely unrelated and comes from the enormous asteroid Ceres—itself a new discovery that had excited the scientific world just two years earlier.
In 1839, a rare cerium mineral (cerium nitrate) was discovered to also contain lanthanum, which was named for the Greek word lanthanein, meaning ‘to lie hidden’. It would prove to be a great choice for the entire lanthanide series! In fact, ‘hidden earth’ elements might have been a better label for what are still referred to as ‘rare earth metals’. As the name implies, they’re not truly rare, they’re just hidden.
‘Rock-loving’ Elements
The Goldschmidt geochemical classification of these oxygen-friendly elements makes them lithophilic, literally ‘rock-loving’ elements that combine readily with oxygen. And, the f-block of ‘rock-loving’ elements is squarely located in between other groups of ‘rock-loving’ elements on the table—groups 1, 2, and 3and, as well as, groups 4 and 5.
As the Earth differentiated, these lithophile elements, chemically, preferred to float with the lighter-weight silicate minerals, oxides, and sulfides near the surface. The iron and the ‘iron-loving’ elements, on the other hand, largely sank into the core.
Common Questions about Understanding Horizontal Groupings in the Periodic Table and the F-block Elements
The lanthanides all behave a lot like lanthanum, even though lanthanum has no f-shell electrons, because they all have the same outer shell configuration as lanthanum.
In 1787, in a small quarry in Ytterby, Sweden, a strange black mineral, named ytterbite, was discovered. A Finnish chemist, by the name of Yohan Gadolin, isolated a substance from that mineral that he called ‘new earth’.
The name Cerium comes from the enormous asteroid Ceres—which itself was a new discovery that had excited the scientific world just two years earlier.
