# How does the state of iodine affect its HOMO–LUMO gap?

My teacher told me that the HOMO–LUMO gap in solid iodine is smaller than that in gaseous iodine.

When I asked about the reason, he told me that it is due to higher interactions in solid iodine as compared to gaseous iodine, which I did not understand. How can the physical state of a compound affect the HOMO–LUMO gap, even though the chemical formula is the same?

• Chemical formula of solid Na and gaseous Na is also the same: Na. But I suppose you aren't surprised that their HOMO/LUMO stuff is quite different. – Ivan Neretin Sep 6 '17 at 8:46

• Molecules may alter their shape in solid state compared to gaseous state. This is less of an issue in $\ce{I2}$.