F Block

 

The basic Bohr model of an atom suggests that atoms in an element have a certain number of electrons that orbit the nucleus in shells, which is good enough for the basic understanding of how atoms function but doesn't truly describe how electrons are bundled in an atom. In reality, electrons are found in orbitals that orbit the nucleus, which in turn are found at discrete energy levels - that is, electrons can only absorb a fixed amount of energy in order to move up into the next energy level, leaving their orbital behind.

There are four different types of orbitals - the s orbital, which contains two electrons; the p orbital, containing six electrons; the d orbital, containing ten electrons; and finally the f orbital, containing fourteen electrons. I've ordered them by increased energy level, and it's important to note only certain atoms will ever have electrons in the f orbital, which look quite convoluted (image source - https://socratic.org/questions/58ca8005b72cff4e5daf7d17) - for me, they're mutated spinning tops.

 

 

 

 

 

 

 

The atoms which have valence electrons in these orbitals are from the lanthanides and actinides - the hidden category of elements, all metals, which are often excluded from the periodic table. This is somewhat for a good reason - the elements are very unstable, with the actinides mostly artificial and all radioactive. They're sometimes called the inner transition elements as well - much like the transition metals which form a large block in the periodic table. It's important to note that not all lanthanides and actinides have electrons in their f orbitals; their physical properties take precedence.

The lanthanides (atomic numbers 57 to 71) are called that because of the first element in the category, lanthanum. The rest of the elements are rather similar to lanthanum, that is . And they make use of their 4f orbitals (4f because they're in the fourth period of the periodic table). The one possible exception is lutetium, which also uses up part of a 4d orbital but which is sometimes included as it has similar properties to the rest of the lanthanides.

These properties are:

• High densities
• Comparatively soft to other substances
• Magnetism
• They are lustrous.
  

What's notable about these elements is that three of them were discovered in the Swedish town of Ytterby - Terbium, Erbium and Ytterbium. They're much like Yttrium in this sense - all four are rare earth elements (a category consisting of the lanthanides, with scandium and yttrium), which are often used in many different ways:

• Cerium is used as a catalyst and in polishing
• Prometheum is a source of beta radiation
• Erbium is a glass colorant and is used in lasers
• Europium is used in LCDs

They're only going to become more popular as low carbon technologies become increasingly popular. There are also many uses for lanthanide compounds for drug delivery when their ions are bonded to ligands, as this paper discusses.

 

The actinides (atomic numbers 89 to 103) meanwhile often reads like a list of famous scientists. Only the first four elements haven't been discovered through lab synthesis, and one of them - uranium - has heavy use in nuclear reactors and weaponry. The rest...you're unlikely to encounter. Much like with the lanthanides, lawrencium's often included though it's not part of the hidden block.

Their main properties are:

• Highly radioactive and can undergo nuclear reactions
  
• High instability
  
• They are rather lustrous
  

The four elements that can be found in traces are actinium, thorium, protactinium and uranium. Actinium and protactinium were first discovered as the products of uranium decay, as are many of the other actinides; decay uranium-238 and you produce plutonium-239, another element commonly used in nuclear industries. Half lives in the actinides' most common isotopes can vary from millions of years to a mere hour, in the case of nobelium. 

There are compounds featuring the actinides, such as uranium dioxide (UO₂), common in nuclear power stations and once as ceramic glazes - uranium compounds can produce vibrant colours, which were obviously attractive to dinnerware and furniture maufacturers before the health risk from radiation became known. A Berlin metro station features uranium tiles to this day, though they're seemingly not harmful to health. Plutonium dioxide (PuO₂) is similar in that it's also a fuel used in power stations, as well as in spacecraft to power the electrics. Meanwhile, americium is commonly used in smoke detectors.

Actinide ions can also react with halide ions to form compounds such as einsteinium chloride (EsCl₃, which can react with hydrogen to form EsCl₂ - both have stubbed Wikipedia pages). I'm not sure what these compounds could ever be used for, but they exist nonetheless.

Most would argue there are only a handful of elements in the f block which you'll come across, and amongst those you're unlikely to be aware you've come across them - and they'd be right.

All images are from Wikipedia; samarium image courtesy of:

https://en.wikipedia.org/wiki/File:Samarium-2.jpg