What is the chemical formula for iodine gas?

• A. I2
• B. I
• C. I3
• D. I-

Answer: (A)

Iodine gas is made of two iodine atoms bonded together, so the formula is I2. This happens because many non-metals, especially halogens, exist as diatomic molecules in their elemental form to achieve a more stable electron arrangement. An individual iodine atom would be highly reactive and not stable on its own, so it pairs with another iodine atom to form a molecule. A triatomic arrangement (I3) isn’t the common stable form for elemental iodine, and I- would be an iodide ion, which is charged and not neutral iodine gas. At room temperature iodine is a solid and sublimates to a purple gas consisting of I2 molecules.