Beforehand Knowledge of an Element's Properties

Pretend that we never discovered, say, Calcium, but had figured out the rest of the Periodic Table. We’d realize there was an undiscovered element fitting in at #20. Would we be able to figure out, based only on the knowledge that it has 20 protons and before handling it by itself, things like its melting and boiling points, the color of its pure form, whether and how it would react to other substances, things like that? Change the example from Calcium to some other element if you’d like.

Isn’t this exactly how gallium and germanium were discovered?

Generally, elements on the periodic table behave like the elements above and below them. Radium and Strontium are both treated by the body as calcium, for instance, and are used in bones – though with terrible effects.

So when they were looking for new elements, they would look for the properties of the elements above them in the table.

That’s one of the reasons the periodic table was such a big thing when Mendeleev created it. It categorized the elements in a way that hadn’t been done before, and that way was systematic enough to be able to predict where new elements would be found and what their properties would be.

There’s a Wiki article on precisely this topic.

One thing worth mentioning. Since the codifying of Quantum Electro-Dynamics and an understanding of how it defines chemistry we can do vastly better than just splitting the difference between the elements above and below in the periodic table. In principle we could predict very precise numbers for all usual chemical attributes. To the point where if someone said “I’ve got a sample with melting point of x” or “I’ve got a spectrum with lines at y and z” we could say “Wow you have some element A”. Even the colour is predictable - although you need to invoke relativity for metals. Colour isn’t something the periodic table would predict.

I don’t think they were discovered that way, but Mendeleev is said to have predicted their properties, after recognizing the principle of periodicity among the elements. For the first two rows of elements known in his time, lithium through flourine and sodium through chlorine, the similarity of the elements in the columns was already obvious once he hit on the right number of columns.

But in the rest of the rows we now know that the transition metals throw the sequence off, as it were. I don’t understand how Mendeleev worked those into his system, but that’s elementary chemistry which I’ve forgotten.

Bolding mine.

“In principle” as you say. The practical problem as discovery was occurring is determining that you have a pure sample of A with those measured properties, rather than a sample of gosh-knows how many elements combined in gosh-knows how many ways. With the result that the sample displays melting point x and spectral lines y & z.

It’s remarkable to me how much difficult work it took in the 1800 & 1900s to tease all this stuff out.

Does this suggest we could have a reasonable guess as to the chemical qualities and properties of, say, Seaborgium, as large amounts of that element might behave if they weren’t insanely unstable? Melting point, bond-formation, etc.?

The transition elements were thought to be part of the same 8 groups. But they were sufficiently different that they were put in subgroups which were signified by the letter B, with the non-transition elements in the A subgroups. You can see how this was done in this labeling of the groups.

But that was after people had had a while to sort things out and after more discoveries subsequent to Mendeleev’s proposal. I found this page about the history of the Periodic Table which shows the original form and some of the predictions Mendeleev made.

This was especially true when they were trying to sort out the Rare Earth elements. Those were difficult because chemically they are very similar. There were a lot of mistaken claims of discoveries of those. The Periodic Table was not useful for these because of the chemical similarity. It doesn’t even predict how many there were, or if it does, it predicts the wrong number. (It would have predicted the right number if atomic number had been known about. But that wasn’t discovered until later; they were going by atomic weight and valency at the time.)


Yep; part of the hunt for ever-heavier elements was in order to check predictions like those. Flerovium, the element under lead, has only been made in tiny amounts, but already enough for some chemical experiments (nowadays we can make chemistry in tinyminimicroscales which would have been unconceivable just 40 years ago).