For 10 points, name this group of elements which includes astatine, bromine, iodine, chlorine, and fluorine.
An SN2 reaction performed in acetone, the Finkelstein reaction, exchanges two of these. The heaviest is made by bombarding bismuth with alpha particles. A silver salt of one of these substances is used in cloud seeding and photography. The four lightest ones are diatomic, and form acids with hydrogen. The only group to have solid, liquid, and gaseous elements at STP, they have seven valence electrons and comprise Group 17 of the periodic table. For 10 points, name this group of elements which includes astatine, bromine, iodine, chlorine, and fluorine.
An SN2 reaction performed in acetone, the Finkelstein reaction, exchanges two of these.
{halogens} [prompt on {Group 17} or {Group VIIA} before mentioned]