Which group of the periodic table is the seventeenth group and includes chlorine, fluorine, bromine, and iodine?

• A. Halogen
• B. Noble gas
• C. Alkali metal
• D. Transition metal

Answer: (A)

Halogens are in the seventeenth column of the periodic table. They all have seven valence electrons, so they tend to gain one electron to complete an octet, which makes them highly reactive nonmetals. In their elemental form, they exist as diatomic molecules—F2, Cl2, Br2, I2—reflecting their tendency to pair up. Their strong electronegativity and preference to form −1 ions help explain why they readily combine with alkali metals to make salts. Chlorine, fluorine, bromine, and iodine are classic members of this group, the halogens. The other groups don’t fit: noble gases are inert and sit in the next group, alkali metals are metals in the first group, and transition metals occupy the d-block. So the best match for those elements is halogen.