By Ron B. Davis, Jr., Georgetown University
Although all of the d-block elements are metals, in some ways their properties couldn’t be much more different. The d-block, for example, contains tungsten, the element with the highest melting point on the entire table. But just six spaces over, we encounter mercury, the only metallic element that melts below room temperature. So, how does one explain these trends?
Let’s compare titanium to osmium. Titanium is only about two thirds as dense as iron. Osmium, on the other hand has a density nearly three times that of iron, and is the densest element on the entire periodic table.
As diverse as some of these properties are, they do trend across the block in very systematic ways. If we apply our understanding of atomic structure, it is very much possible to explain these trends.
In order to do so, first, let’s take a look at the atomic radius.
The Atomic Radii of the D-Block Elements
For the s- and p-block elements, there is a strong and orderly trend that the atomic radius gets smaller as we move across from left to right, and increases steadily within a group as we move downward.
But when we examine the d-block elements, we see something a little different. Their atomic radii do decrease gently and pretty consistently as we move from left to right. This trend only begins to deviate as we reach groups at the end of the block, where the d-subshell is beginning to fill up completely.
But as we move down through a group, we see that the atomic radii of elements in periods 5 and 6 are almost identical.
Size and Valence Shell Configurations
The reason for this is that we have to account for the f-subshell filling that takes place in period 6 prior to the d-subshell filling. So, on the one hand, the nuclei of transition metals from rows 6 have far more positively-charged nuclei for pulling the electrons in than do the smaller transition metals. And on the other hand, the added f-subshell electrons are not very efficient at screening the valence electrons from all those nuclei.
Similar size and similar valence shell configurations also explain why row 5 and row 6 d-block elements within the same group are so often are found together in nature.
Niobium and tantalum, for example, were discovered just one year apart using the same source mineral. And zirconium and hafnium are so similar that hafnium actually wasn’t discovered for more than a century after zirconium.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
As we move down and right across the d-block, increasing the mass of the nucleus also contributes to increased density. This explains why sixth row elements like osmium, iridium, platinum and gold have very high densities, while the early third row transition metals like titanium and vanadium have some of the lowest densities for the entire block.
When it comes to melting points, another strong trend emerges in the d-block as well. Firstly, virtually all of these metals have relatively high melting points, with a sharp drop-off only in the last two columns of the block. Secondly, there is a clear vertical trend within the d-block of increasing melting point as we move down a column, and this makes sense.
In order to understand this let’s take the group 6 column of chromium, molybdenum, and tungsten. Each time we step down to the next row, we are adding an extra energy level of electrons, which are better screened from the nucleus, making those valence electrons more available for metallic bonding. Stronger metallic bonds mean a higher melting point.
Trend across Rows
Tungsten is a valued metal for just this reason. Its high melting point means that tungsten filaments can tolerate the extreme heat released when they generate light in incandescent light bulbs without melting the filament.
Now let’s turn our attention to the trend across rows. This is a very unusual looking trend, with melting points at first increasing as we move left to right across a period, but only to about the half-way point. After this, melting points seem to generally go down as we continue to move across.
At first, moving left to right across the d-block results in the addition of more unpaired electrons to the atom. Each additional unpaired electron means a stronger metallic bond that can form.
So as we progress through the d-block elements’ groups 3, 4 5 and 6, there is a well-defined trend of increasing melting points. But at Group 7 something else happens, particularly in the top row of the d block, where manganese has a much lower melting point than chromium. If we look closely, we can see a similar feature for technetium and rhenium in periods 5 and 6 as well.
What’s so special about Group 7 of the d-block elements? Why are the metallic bonds in these elements weaker than their predecessors on the table? This is because an unusual degree of stability can be achieved by half-filled subshells. Manganese atoms enjoy a bit of extra stability when they retain all of their electrons, thanks to being a 4s2-3d5 metal.
But once we move on to Group 8 and beyond of the d-block elements, the half-filled subshell effect is no longer in play. Melting points seem to decrease as we continue across the d-block. This is because of unpaired electrons. On the left side of the block, each new d-electron means one more d-electron available for metallic bonding.
By contrast, for the second half, the d-subshell is getting crowded, its orbitals are taking on more and more electron pairs, making fewer d-orbital electrons available for metallic bonding.
By the time we get to the zinc group, we have closed s- and d-subshells with no unpaired electrons at all. Here, the d-subshell contribution to metallic bonding is completely over, making zinc-group elements act more like ‘weak metals’.
Common Questions about the Diverse Properties and Trends of D-block Elements
Tungsten‘s high melting point means that its filaments can tolerate the extreme heat released when they generate light in incandescent light bulbs without melting the filament.
There is a clear vertical trend within the d-block elements, of increasing melting point as we move down a column.
The reason why there’s a rising trend, followed by a falling trend, within each single row of the d-block is because of unpaired electrons.