Although I2 is the stable form of iodine at low temperatures, at high temperatures the molecule can dissociate:

Upper I subscript 2 (g) double-headed arrow 2 upper I (g).

What is the equilibrium constant expression for this system?
K subscript e q equals StartFraction StartBracket upper I EndBracket superscript 2 over StartBracket upper I subscript 2 EndBracket EndFraction.
K subscript e q equals StartFraction StartBracket upper I EndBracket over StartBracket upper I subscript 2 EndBracket EndFraction.
K subscript e q equals StartFraction StartBracket upper I subscript 2 EndBracket over StartBracket upper I EndBracket superscript 2.