The maximum wavelength that can break a Cl-Cl bond is 494 nm.
Energy to break 1 Cl-Cl bond
Energy = (242 kJ/1 mol) × (1000 J/1 kJ) × (1 mol/6.022 × 10²³ bonds)
= 4.019 × 10⁻¹⁹ J/bond
Wavelength of photon
E = hf = (hc)/λ
λ = (hc)/E = (6.626 × 10⁻³⁴ J × 2.998 × 10⁸ m·s⁻¹)/(4.019 × 10⁻¹⁹ J) = 4.94 × 10⁻⁷ m = 494 nm
