By Ron B. Davis Jr., Georgetown University
Of all the elements in the seventh row of the periodic table, only two elements have isotopes long-lived enough to qualify as primordial elements. These are the elements thorium and uranium. These have isotopes that decay with half-lives in the billions of years, leaving a fair amount of them in and on the Earth today.
At the very bottom of the periodic table, all the elements are radioactive. That is to say that across the whole of row seven of the periodic table, not a single element has even one stable known isotope.
This is not surprising, considering that after lead, every element, starting with element 83, bismuth, exists only in a radioactive form. And yet, whereas bismuth’s most stable isotope has a half-life about one billion times the age of the universe, the overall trend is toward shorter half-lives, with more intense emission of radiation. This pervasive radioactivity and instability of elements beyond lead means, in many instances, that it is the nuclear reactions of these elements that interest us more than their chemical reactions.
Thorium and Uranium
It is interesting to note that, though all of the seventh-row elements may have formed in the stellar explosion that seeded our solar system at its birth, only thorium and uranium remained for us to discover.
One might assume that thorium and uranium being notoriously radioactive would mean they are also rare—after all, being radioactive means some of their atoms are constantly decaying away, transmuting into atoms of other elements. And it’s true that most elements from row seven of the periodic table are extremely rare because any of their atoms that existed at the start of our solar system have long-since decayed.
But in the case of thorium and uranium, one would be wrong.
As a proportion of Earth’s crust, thorium and uranium are about as common as lead or tin. Uranium is about 40 times more common than silver and 500 times more common in our environment than precious metals—gold, platinum, or palladium. Thorium, in part because it decays even more slowly, is roughly three times more abundant than uranium.
Thus, clearly, thorium and uranium are more abundant than their radioactivity might suggest. This is because they are very weakly radioactive, especially in comparison to most other elements in their vicinity on the table. Over 99.98% of thorium on Earth occurs as its most stable isotope, thorium-232, which has a half-life of 14 billion years! Similarly, over 99% of uranium takes the form of U-238, with a half-life of almost 4.5 billion years—that half-life is roughly the same as the entire ‘life’ of Earth so far.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
The Lanthanide Series
Both thorium and uranium are a part of the actinide series. Interestingly, because the actinide series is just below the lanthanides (later called lanthanoids), beginning in 1945, the corresponding terms ‘actinide’, and later ‘actinoid’, became a common way to refer to this series of elements.
Just as with the lanthanide series, here too we have group of 15 elements, but only 14 slots in the block. Thus, we have the same debate over which element of the group does not belong. Does Actinium, at the front of the group, not belong under scandium and yttrium? Or, since the f-shell is already full at element 102 Nobelium, does element 103 lawrencium take that spot?
Once again, there are exceptions to the otherwise orderly Aufbau principle that explain why more recent tables tend to put all 15 together.
Similarities with the Lanthanides
The actinides series on the bottom row, overall, do share some similarities with the lanthanides. Most notably, they involve the active filling of the huge f-subshell, where there’s no pairing up of electrons in f-orbitals until each of the first seven orbitals gets at least one electron. Hence, elements toward the middle in both groups have lots of unpaired electrons.
It is also because of these unpaired f-subshell electrons, that all actinides, that scientists have been able to collect and study, have proven to be paramagnetic. It refers to the fact that they are attracted to a magnetic field, without themselves being permanently magnetic.
Radioactive Decay More Rapid
However, in many important ways the actinide series of elements is also quite different than the lanthanides series.
The big difference is that now there’s radioactivity everywhere. While earlier, promethium was the only lanthanide element with no stable isotope, now the script is flipped. There are now no elements with stable isotopes on the bottom row of the f-block.
Indeed, once we pass beyond element 92 uranium, radioactive decay becomes progressively more and more rapid. None of the remaining elements in the entire table, from atomic number 93 to 118, has even a single primordial isotope that can be obtained from natural sources. This is because all of those so-called ‘trans-uranic’ elements, from neptunium to oganesson, all contain such vast numbers of protons in their nuclei. The natural repulsion of all of those positive charges soon leads to radioactive decay, or even fission, of all their known isotopes.
Actinoids and the Later Actinides
In conclusion, it is worthwhile to remember that, the other members of the first six actinoids, actinium through plutonium, were probably produced during whatever stellar event generated the thorium and uranium we still have today. But due to far, far shorter half-lives, all of those atoms have long since decayed, leaving nuclear reactions of thorium and uranium as the only means by which they naturally appear.
When it comes to the later actinides, they likely do not occur naturally in any meaningful quantity. There is little evidence to suggest that these so-called trans-plutonic actinides, with half-lives ranging from a few thousand years, for americium, to just four hours for lawrencium, ever existed in more than the most miniscule amounts until they were produced synthetically in a laboratory.
Common Questions about Uranium and Thorium
Most elements from row seven of the periodic table are extremely rare because any of their atoms that existed at the start of our solar system have long-since decayed.
Thorium and Uranium are more abundant than their radioactivity might suggest. This is because they are very weakly radioactive.
The first six actinoids, actinium through plutonium, were probably produced during whatever stellar event generated the thorium and uranium we still have today.