Hello, friends!  Welcome to Sciency Words, a special series here on Planet Pailly where we take a closer look at the definitions and etymologies of scientific terms.  Today on Sciency Words, we’re talking about the word:

CORONIUM

Here on Sciency Words, we usually talk about scientific terms that are relevant and useful in modern science, but sometimes I like to draw attention to scientific terms that didn’t make it.  I think it can be helpful to learn about how and why words drop out of the scientific lexicon.  So today, we’re going to talk about coronium, a chemical element that we now know does not exist.

Definition of coronium: A chemical element that scientists in the late 19th and early 20th Centuries thought existed based on a mysterious green emission line detected in the Sun’s corona.  At least one very prominent scientist (Dmitri Mendeleev) believed coronium to be an element lighter than hydrogen, with chemical properties similar to helium and argon.

Etymology of coronium: In 1869, American astronomers Charles Augustus Young and William Harkness independently detected a green emission line in the Sun’s corona during a solar eclipse.  In 1887, Professor A. Grünwald proposed the name “coronium” for whatever chemical substance caused that green emission line.  Since this unknown substance was first detected in the Sun’s corona, coronium seemed like an obvious name.

The “discovery” of coronium came right on the heels of the discovery of helium, and the story of these discoveries was eerily similar.  Scientists observe a solar eclipse.  A strange, new emission line appears in Sun’s spectrum, as measured using a spectroscope.  This emission line is (or seems to be) the first evidence of a newly discovered chemical element.

Dmitri Mendeleev was initially skeptical about both helium and coronium, because he couldn’t find places for them in his periodic table of the elements.  Toward the end of his life, however, Mendeleev tried to shoehorn these elements, along with several others, into his theories by adding a “group zero” to the periodic table.  Each group zero element is lighter than the group one element it sits next to—for example, argon is lighter than potassium, neon is lighter than sodium, helium is lighter than lithium… and coronium ended up sitting next to hydrogen, indicating that coronium is an element lighter than hydrogen.

Mendeleev was a smart man, but he was wrong about group zero.  After some reshuffling of the periodic table, most of the group zero elements were moved to group eighteen (a.k.a. “the noble gases”), and in the end, it turned out there really was no place for coronium.  No element lighter than hydrogen exists.

So what caused that anomalous green emission line in the Sun’s spectrum?  Turned out it was iron.  In the 1930’s, German and Swedish astronomers Walter Grotian and Bengt Edlén discovered that a form of super-hot, super-ionized iron gives off an emission line at 530.3 nm—an exact match with the 530.3 nm green emission line found in the solar corona.  Without the power of the Sun (or the power of modern laboratory equipment), iron doesn’t get hot enough or ionized enough to reveal that part of its spectrum.  As a result, scientists in the late 1800’s couldn’t have known what that strange, green emission line was.

Coronium is a Sciency Word of the past, from a time when the spectroscope was a relatively new scientific instrument and the periodic table was still a work in progress.  We no longer need to imagine there’s an exotic chemical element found only in the Sun’s corona, not when super-ionized iron explains that green emission line in the Sun’s spectrum just as well.

WANT TO LEARN MORE?

Here’s an interesting article about Dmitri Mendeleev and his mistakes, including his mistakes about coronium and the “group zero” elements.  For anyone involved in science education, this article makes a compelling case about why teaching the history of science is so important, with an emphasis on showing how scientists don’t always get it right on the first try.

I also want to recommend this book, simply titled The Sun.  It is full of cool and useful space facts that I had never read about before anywhere else (including the false discovery of coronium).  The Sun is part of a series called Kosmos, and I highly, highly, highly recommend this series to anyone who loves space.

And lastly, here’s a link to A. Grünwald’s 1887 paper where he first proposed the name “coronium” for a “hitherto unknown corona-substance.”