[Nemesisof99]

At a certain temp and total pressure 1 atm, iodine vapours contains 40% by volume of I atoms.
I_2(g)--->(reversibly)   2 I(g)
Calc. Kp. What does the darkened line actually mean? 

[BetaAmyloid]

iodine vapours contains 40% by volume of I atoms.
I_2(g)--->(reversibly)   2 I(g)

I₂ molecules and I atoms reversibly interchange between molecule-atom. I atoms constantly change into I₂ molecules and I₂ molecules constantly change into I atoms. Iodine vapors consist of 40% I atoms. This means that 40% of the vapor particles are I atoms and the other 60% are I₂ molecules.