What is the chemical formula of iodine as a diatomic molecule?

• A. I2
• B. I
• C. I3
• D. IO

Answer: (A)

Iodine in its elemental form exists as diatomic molecules because atoms of this non-metal prefer to pair up to achieve a full outer shell. Each iodine atom has seven valence electrons; by sharing one pair with another iodine atom, both atoms reach eight electrons in their outer shell, forming a stable covalent bond. This yields the diatomic molecule I2, which is the typical form of elemental iodine (solid at room temperature, subliming to a purple gas).

The other possibilities aren’t the elemental iodine form: a single iodine atom isn’t stable as a free molecule under ordinary conditions, a tri-iodine species would involve three iodine atoms rather than two, and iodine monoxide is a compound that contains oxygen, not just iodine in its diatomic elemental state.