Completing Mendeleev’s Periodic Table


By Robert M. Hazen, Ph.D.George Mason University

Dmitri Mendeleev forced the most unambiguous groups of similar elements into vertical columns in the periodic table. He arranged the elements left-to-right and top-to-bottom by increasing weights. So he had a side-to-side and a top-to-bottom arrangement based on weight. Scientists readily accepted the concept of the periodic table, not only because it systematized so much of what was known but because it made very specific testable predictions about what wasn’t known.

The periodic table of elements.
Each column in the periodic table contains elements with similar properties. (Image: Maryna Yakovchuk/Shutterstock)

Arranging the Periodic Table

To arrange the periodic table, Dmitri Mendeleev put lithium, sodium, potassium, and rubidium into one column. Beryllium, magnesium, calcium, and barium are in another column. Chlorine, bromine, iodine, that’s going to be in a third column, and so forth. Thus, sodium was next to magnesium, potassium was next to calcium, and so forth. He also left spaces for elements that appeared to be missing. 

One apparently missing element lay on the fourth row between calcium and titanium:  that’s scandium, element 21. It was discovered in 1876, just as Mendeleev predicted. Another gap seemed to occur in the column headed by carbon. The element below that is silicon, and above the tin, there was a blank. Germanium, element 32, was discovered in 1886, and it filled that blank. New elements continued to fill out and expand the periodic table.

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New Techniques to Discover Missing Elements

An image of flame spectroscopy in a lab.
Flame spectroscopy was used to discover many heavy elements. (Image: Rabbitmindphoto/Shutterstock)

For more than a decade between 1863 and 1875, there were no new elements discovered. The old technique of electrolysis had been pushed to the limit. Blowpipe analysis was no longer effective to find new elements.

It was the development of flame spectroscopy, by which elements could be identified by their distinctive bright-line spectrum when they were heated up to an incandescent glow, that triggered another wave of element discoveries in the mid-1870s through the 1890s.

Each new element could be identified by its unique fingerprint when it glowed. Recall that every element has its own distinctive separate electron energy levels. Thus, each element, when it’s heated to a high temperature, has its own unique set of wavelengths that are emitted as electrons are heated up and dropped back down. 

And those can be used as a distinctive line spectrum to distinguish new elements. More than two dozen rare elements, 57 through 71, were discovered in this way in the late-19th century. These heavy elements are chemically so similar that they always occurred lumped together in minerals in the natural world. They all have very similar chemical properties but can be distinguished and isolated using flame photometry.

William Ramsay and the Discovery of Inert Gases

Between 1895 and 1898, many of the inert gases were discovered. And these discoveries were made by the Scottish chemist William Ramsay, who lived from 1852 to 1916. He used spectroscopy to identify five of these inert gases. Sometimes they’re called noble gases. These are elements that are so unreactive that they don’t bind to anything. 

They occurred only as isolated gas atoms in nature. Mendeleev’s table predicted these to be elements 2 and 10 and 18. These are now known as helium, neon, and argon. These previously unseparated elements added a whole new column to the periodic table, and that column was also amplified with krypton and xenon, which are part of these groups. Ramsay, by the way, received a Nobel Prize and many other honors for this work in adding a whole new column to Mendeleev’s table.

Learn more about phase transformations and chemical reactions.

Attempts to Isolate Fluorine

Mendeleev’s periodic table was incomplete in other ways as well.  He had to include the element fluorine, which was well known to be a major component in many common minerals, especially in calcium fluoride, the mineral fluorite. Yet, no one had been able to isolate fluorine, and this effort became a major challenge in the late-19th century—the effort to isolate the element fluorine, to show what the properties of fluorine might be. 

Mendeleev’s table provided the researchers with some useful hints. Fluorine was at the top of the column containing chlorine, bromine, and iodine. These are all extremely reactive nonmetals, and they can be produced by the electrolysis of sodium or calcium compounds. The periodic properties suggest that fluorine would require an extremely high voltage. It would be extremely reactive—a very, very dangerous substance. 

And so it was that early attempts to isolate fluorine with electric current were disastrous. They caused injury and death to many chemists. Strong batteries could, indeed, separate fluorine from calcium. For example, with calcium fluoride, the fluorite mineral, fluorine can be separated. But the gas turns out to be so reactive that it attacks and destroys tissues almost immediately. 

Learn more about the properties of materials.

Henri Moissan

An image of Sorbonne Université Professor, Henri Moissan.
Henri Moissan successfully isolated fluorine gas. (Image: Bibliothèque interuniversitaire de Santé, Licence Ouverte/Public domain)

So the isolation of fluorine became a great challenge in the late-19th century. In 1886, French chemist Henri Moissan successfully isolated the element after almost three years of effort. He passed a strong electric current through a solution of fluorine salts, just as his predecessors had done. And sure enough, the fluorine gas bubbled up. 

But the thing he did differently was he built his entire apparatus out of fluorite, which is probably the only substance that will contain the fluorine gas because it is already fully saturated with fluorine. He won the Nobel Prize for this work. And even so, even though he was able to do this, he said that it probably took 10 years off his life, this successful effort to isolate fluorine. It was a great chemical challenge, but the people who did it really paid a price.

Common Questions about Completing Mendeleev’s Periodic Table

Q: How were elements 57-71 in the periodic table discovered? 

More than two dozen rare elements, 57 through 71, in the periodic table, were discovered in the late 19th century. They all have very similar chemical properties and can be distinguished and isolated using flame photometry. Each element has a distinct bright-line spectrum from which it is identified.

Q: Why were initial attempts to isolate fluorine unsuccessful? 

Fluorine gas is very reactive and early attempts to isolate fluorine with electric current were disastrous. It caused injury and death to many chemists.

Q: Who was Henri Moissan? 

Henri Moissan was a French chemist who was finally able to safely separate the element fluorine and fill in the gap in the periodic table. To do this, he passed a strong electric current through the fluorine salt solution to release the gas. To prevent the fluorine from reacting, he made his entire apparatus from fluorite.

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