For 10 points, name this lightest of the halogens, an element with atomic number 9 and symbol F.
In organic synthesis, an acetic acid derivative containing three atoms of this element is useful due to its extreme acidity. A compound exhibiting pentagonal bipyramidal geometry contains 7 atoms of this element bonded to iodine--that compound is stable since this element forms very small ions. A notably square planar compound consists of a xenon atom bonded to 4 atoms of this element.  An acid consisting of hydrogen and this element has the ability to etch glass. The Pauling scale assigns the highest value of electronegativity to this element, whose 19 isotope is used in NMR. For 10 points, name this lightest of the halogens, an element with atomic number 9 and symbol F.
In organic synthesis, an acetic acid derivative containing three atoms of this element is useful due to its extreme acidity.
fluorine