By Ron B. Davis, Jr., Georgetown University
Dmitri Mendeleev’s table divided elements into eight groups, using eight Roman numerals, based on how elements reacted with hydrogen and oxygen. The table did a pretty good job of clustering the alkali metals and alkaline earth metals of the s-block and p-block. And yet, elements from the d-block of the modern table brought to attention some shortcomings of Mendeleev’s system.
Mendeleev’s group seven contained not only the nonmetal halogens fluorine, chlorine bromine and iodine, but also the decidedly metallic element manganese. Manganese has many properties that make it quite different from the halogens, yet its atomic mass of 55 suggested to him that it needed to be included in the same group, between chlorine at 35, and bromine at 80.
His group six contained the nonmetals oxygen, sulfur, selenium and the metalloid tellurium; but Mendeleev’s organizational scheme also meant that group six should contain the metals chromium, molybdenum, tungsten and uranium.
However, in Mendeleev’s group, tungsten seems a particularly egregious trespasser, with its staggeringly high melting point, hardness and electrical conductivity. How could this element possibly belong in the same group as oxygen or sulfur?
Mendeleev seemed to be acutely aware of this imperfection, as he attempted to inject more order into his table in 1871 by creating additional periods that contained these uncooperative elements.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Defying Proper Organization
Mendeleev further highlighted this by right-justifying for periods containing nonmetals and left-justifying for those that did not. But groups that contained such motley mixtures of elements weren’t the only hint that Mendeleev’s table was too simple. Certain rows of his table also seemed to have too many elements!
In order to preserve periodicity, iron, cobalt, nickel, copper and others had to be clustered on the right side of the table in Mendeleev’s group eight.
While certain elements seemed to have a tidy home in his table, a dozen elements defied proper organization. Even in 1906, Mendeleev’s final offering of his periodic table, still lacked a d-block.
In 1905, though, Swiss chemist Alfred Werner published a much more modern-looking table. Werner chose to use the largest period of elements known at the time as the basis for the number of columns. This was period six starting with cesium.
Werner’s table was the first to depict the elements of the d-block and f-block separated out from the rest, and also one of the first to place the noble gases at the right edge. But this offering received little attention in its day. The discovery of electron subshells and orbitals were still 20 years in the future.
Characterizing Electron Configuration
But ever since Moseley and Schrodinger’s scientific contributions deciphered the structure of the atom, we have known the reason Mendeleev’s table falls short—atoms with substantially different electron configurations can have similar valence. Sorting every element based only on its valence just was not discerning enough to characterize each type of electron configuration.
The d-block made it clear that one really needs to look inside the electron cloud. There are deeper layers of the electron configuration that matter for these elements—just as much as their outer shell.
And when we do consider the full electron configuration of atoms—not just the outer shell—as the defining characteristic that places them into a column, there are not just eight groups of elements. There are 18 groups of elements: that’s 2 plus 6 plus 10, spanning the s-, p-, and d-blocks of the table.
Cracking the Periodic Table
Although Mendeleev’s table doesn’t look much like the modern periodic table in its overall shape, it does actually come very close to cracking the code of the periodic table. One can slightly rearrange the table and clearly see why.
To begin with, Mendeleev’s 1871 periodic table already has the benefit of gallium and germanium but with respect to his lanthanide and actinide elements, he actually got a few of those wrong.
Wrapping the Series
Mendeleev wrapped some of his series at fluorine, chlorine, bromine, and iodine, the halogens. Although that makes a good bit of sense since he didn’t have noble gases as his marker for wrapping those series, but notice that there are additional series that are created in which he didn’t have a halogen as a marker to wrap that series. And so, he ended up with his group VIII which contained lots of additional elements.
But he chose to wrap those at the coinage metals, copper, silver, and gold. And he did that because it was known that these elements could combine in oxygen, with oxygen rather in the same ratio as some of these other Group elements.
However, instead of wrapping the table, let’s take these series, these additional series that were generated by wrapping the table at those metals, and see what happens when we put them back together.
Close to Modern Periodic Table
In order to do that, we’re going to have to make a little extra room and expand those series out. Thus, we’re going to connect copper to copper, silver to silver, gold to gold to create fewer but longer series of elements. And when we do that, we start to see something that looks more and more like the modern periodic table as we go.
In fact, once we’ve done this, all we really need to do is make the commitment to shift the p-block elements over to keep them in line and we see looks awfully similar to the modern periodic table with simply six series in it rather than in the layout as 12.
Thus, now one can clearly see why today’s periodic table is more spread, having many more groups and many fewer periods than some of the earliest versions and how Mendeleev’s table comes very close to cracking the code of the periodic table.
Common Questions about D-block Elements and Mendeleev’s Periodic Table
Dmitri Mendeleev’s table divided the elements into eight groups, using eight Roman numerals, based on how elements reacted with hydrogen and oxygen.
Alfred Werner’s table was the first to depict the elements of the d-block and f-block separated out from the rest, and also one of the first to place the noble gases at the right edge.
Although Mendeleev’s table doesn’t look a whole lot like the modern periodic table in its overall shape, it does actually come very close to cracking the code of the periodic table.