fluorine

/ˈflɔːɹiːn/
noun
  1. 1

    The chemical element (symbol F) with an atomic number of 9. It is the lightest of the halogens, a pale yellow-green, highly reactive gas that attacks all metals.

  2. 2

    A single atom of this element.

    an octahedron of fluorines

Source: https://en.wiktionary.org/wiki/fluorine