The d-block elements of the periodic table are also sometimes casually referred to as the ‘transition metals’. The Aufbau order predicts the fourth-period elements all begin with a 4s subshell that contains electrons, but as we make our way across the table from scandium to copper, each additional electron is placed not in the fourth energy level, but in the 3d-subshell—that is, on the third energy level.
Irving Langmuir and Charles Bury
In 1919, Irving Langmuir published a famous paper entitled The arrangement of electrons in atoms and molecules. In his paper, Langmuir asserted that each principal energy level must fill completely before the next is populated.
But, English chemist, Charles Bury, published a paper challenging Langmuir’s postulate in 1921. Bury proposed that certain properties of elements in what he called the ‘long periods’ could be better explained if elements did, indeed, start to place electrons in a higher energy level, then ‘transition’ back to filling in an interior shell with even more electrons
Bury suggested that this was exactly what was happening in elements from scandium to zinc, and he was correct!
Like Werner’s more modern table, the Aufbau order’s prediction about the fourth-period elements, was an amazing bit of insight years before Schrodinger confirmed the existence and structure of subshells and orbitals.
But as our understanding of atomic structure has continued to evolve, the definition of ‘transition element’ has also changed slightly. The 1997 IUPAC ‘goldbook’ defined the term ‘transition element’ with a broader, two-pronged definition, as ‘an element…whose atom has an incomplete d sub-shell—without assuming any particular fill order or an element which can give rise to cations with an incomplete d sub-shell.
Putting the D-block Elements to the Test
Thus, an element that does not itself have an incomplete d sub-shell may nonetheless be considered a transition element, if it gives rise to an ion with that property.
So, let’s put the d-block elements to the test using the first of these two qualifications—the ground-state configuration.
To begin with, let’s consider the top of the d-block, that those elements that have 3d subshells, that are filling. One can notice that the 3d and 4s subshells are very close in energy with the 3d only slightly higher. This is important. Let’s fill this region using those elements where those particular subshells that are operative, which would be our 4s and our 3d elements potassium through zinc. We’re going to line them all up.
Electrons in the D Subshell
What happens is that we progressively add one additional electron for each step over on the periodic table and thus, all of them will end up having an argon core. Once we start filling them, potassium, which has a single electron above the argon core, is going to go into the lower energy subshell into the 4s1. As we step again to calcium, we still have space in the 4s subshell, therefore, calcium gets a 4s2 electron configuration.
Moving on, scandium now has a full 4s and also needs to put one more electron in, so it’s going to add that electron to the 3d, giving us 4s2 3d1. As one can imagine, this process is going to continue. We’re going to get additional electrons in the d subshell as we continue stepping across this row until we reach chromium.
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Now at chromium, one might think that it’s 4s2 3d4. However, a half-filled 3d subshell brings with it an additional sort of energetic benefit. Hence, actually, chromium does not assume the 4s2 3d4 configuration in its ground state. Instead, the 3d being so close in energy to the 4s is able to steal an electron away, making this a 4s1 3d5 configuration, an apparent violation of the Aufbau order.
And then, things get back to normal again for a while. We get to manganese, we again have a 4s2 now 3d5. With iron, we get ourselves a 4s2 3d6. And, again, the progression seems to be going along nicely through cobalt and nickel until eventually we reach copper, which, again, the Aufbau order predicts should be a 4s2 3d9.
Violation of the Aufbau Order
But there’s also a special energetic benefit associated with a full d subshell, completely full, and this is accessible to a copper atom. It simply steals another one of those 4s electrons away and becomes a 4s1 3d10 in its ground state, another violation of the Aufbau order.
From here on, we finally reach zinc in which both of these subshells are completely close at 4s2 3d10. Thus, we have the situation here where chromium and copper, because they have the ability to get half-full or completely full d subshells by snatching one electron away from the 4s, actually fill in an order other than that predicted by the Aufbau principle.
Therefore, when we consider the entire d-block, what we find is that most of the exceptions to the Aufbau filling order occur in Group 6 and 11.
Not only do chromium and copper exhibit this behavior, so do molybdenum, silver, and gold. Oddly though, tungsten beneath molybdenum does not exhibit this kind of behavior because of the difference between the s and d subshell energies for that particular nucleus.
Consequently, so far we’ve got our five exceptions to the Aufbau filling order that we already predicted using filled and half-filled subshells. There’s also one last additional violator of our Aufbau principle, and that is the element platinum. Platinum, takes on a 6s1 4f14 5d9 configuration and along with gold, these larger elements do seem to be exhibiting some unusual behaviour.
Thus, in conclusion, the first part of the IUPAC rule locks in groups 3 through 10, scandium through nickel. Groups 11 and 12, however, don’t make the first cut, but the debate over where to draw the line on ‘transition metals’ continues.
Common Questions about D-Block Elements and the Aufbau Order
In his paper, Irving Langmuir asserted that each principal energy level must fill completely before the next is populated.
The Aufbau order’s prediction about the fourth-period elements, was an amazing bit of insight years before Schrodinger confirmed the existence and structure of subshells and orbitals.
An element that does not itself have an incomplete d sub-shell may nonetheless be considered a transition element, if it gives rise to an ion with that property.