A rare earth element or a compound containing two identical atoms or groups bound together.
From Greek 'didymos' (twin), reflecting that the element was often found in pairs or occurred alongside similar elements, as with didymium before it was separated into distinct lanthanide elements.
Didymium was chemically identified as a single element in 1841, but scientists later discovered it was actually a mixture of two rare earth elements (praseodymium and neodymium) that couldn't be separated with 1800s chemistry—a humbling reminder that 'elements' aren't always what they seem.
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