The longest wavelength of light capable of breaking a single (C-CI) bond in Cl2 is ifCl−Cl(g)⟶2Cl(g); ΔH=242 kJ mol−1

ΔH=242kJmol−1=242×103Jmol−1=242×103JN0J molecule −1=242×103J6.02×1023J per (Cl−Cl) bond  Also ΔH=hv=hcλ∴ λ=hcΔH=6.62×10−34Js×3×108ms−1×6.02×1023242×103J=4.94×10−7m=494×10−9m=494nm