Fluorine is the most reactive chemical element and reacts even with glass and some noble gases.
Why it is true
It is so aggressive that several chemists were injured or killed trying to isolate it. The Frenchman Henri Moissan finally managed it in 1886, earning him the Nobel Prize in Chemistry in 1906. In small amounts, its compounds protect teeth in toothpaste.
Source: Royal Society of Chemistry
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Related facts
Francium is so unstable that there is likely less than 30 grams of it in the Earth's crust at any given moment.
Osmium is the densest natural element: a litre of it would weigh more than 22 kilograms.
Astatine is the rarest element in the Earth's crust.
All 118 elements of the periodic table exist naturally on Earth.
