A Side of Science: The Periodic Table

In my latest video, I talked about the Periodic Table and how, as an astronomer, I see the table and our cosmic connection to those elements. I picked this topic because 2019 was named the International Year of the Periodic Table to mark the 150th anniversary of Dimitri Mendeleev publishing a table organizing the elements into families.

One question I had is why is Mendeleev’s table the one we credit for finally bringing the elements together? He wasn’t the first to try, so what was different about his attempt?

Books have been written on the periodic table, so I’m not going to do it justice in a short blog post. But I’ll highlight a couple of things that I found.

One of the big things that set Mendeleev’s apart from other is that he used properties of the elements – like melting point, density and metallic character – to try and make sense of them. Based on this, he ended up leaving a few gaps in his table (denoted as question marks with approximate atomic weights in his published table). Essentially, he was predicting that there were elements that hadn’t been found yet.


Mendeleev, 1869. “An experiment on a system of elements based on their atomic weights and chemical similarities.” (translated)

From listening to a 99% Invisible podcast on the Periodic Table, I learned that the first element found after Mendeleev published his table was gallium, which was discovered by French chemist Paul Emile Lecoq de Boisbaudran. Initially de Boisbaudran had reported a density for this newly-found element that was inconsistent with Mendeleev’s prediction. Mendeleev wrote a letter telling de Boisbaudran to check his result – a bold move that paid off. The actual density was, indeed, close to what Mendeleev had predicted.

The other thing Mendeleev did with his table was swap the order of a couple of elements, rather than going in strict order of their atomic weight. In particular, he swapped iodine (denoted “J” in his table) and tellerium (“Te”). He did this because those particular elements fit into his organization better than if he kept them in order of atomic weight.

In the end, his ordering on those elements was the correct one. We know now, that the overall ordering is driven by the atomic number, not the atomic weight.

What’s the difference? Atomic weight is an average of the number of protons AND neutrons in all variations of a particular element, which includes isotopes of an element that have different numbers of neutrons. Atomic number is the number of protons in the nucleus – this is what defines an element. So, Mendeleev was, in a way, predicting the atomic number.

So, while the table has gone through a few modifications since Mendeleev first published it (getting turned on its side, for one thing), his is the one that is the basis for the periodic tables that hang on the walls of every science classroom. (And maybe even on a filing cabinet in my office….)

And, in case you missed my video on the periodic table of the elements, and connecting it to the game Century: Golem Edition, be sure to check it out:

Resources and additional reading:

Leave a Reply

Your email address will not be published.