The energy required to break one mole of Cl−Cl  bonds in Cl2  is  242  kJ  mol−1. The longest wavelength of light capable of breaking a single Cl−Cl bond is

Energy  required  for  1 Cl2  molecule                        =242 ×  103NA JE=hcλ     or    λ=hcE=6.626 × 10−34 × 3 × 108 × 6.02  ×  1023242 × 103=494 × 10−9  m=494  nm