Mendeleev identified coronium as the heavier of two hypothetical elements lighter than hydrogen.
He proposed these unusual substances after incorporating the newly recognized noble gases into his periodic ideas. The lighter hypothetical substance was associated with “ether” or later with newtonium, while the heavier one was called coronium. Mendeleev imagined coronium as an inert, extremely light gas with an atomic weight below hydrogen’s reference value of 1.
The name came from an unexplained spectral line observed in the Sun’s corona. In the nineteenth and early twentieth centuries, that line was attributed to a supposed new element, just as helium had first been identified spectroscopically in the Sun before being found on Earth. Mendeleev connected the solar-corona line with his proposed ultra-light noble gas.
Coronium was not discovered as a genuine chemical element. The famous green coronal line was eventually explained as emission from highly ionized iron, especially Fe XIV, rather than from a new substance. Thus coronium is historically important as a prediction, but it is not part of the modern periodic table.