see attach

wavelength(l)=4500 angstrom= 4.5*10^-7 metre
then calculate energy for this one quantum of radiation using,
E=hc/l(wavelength in meter)
Then multiply this value by avogadro number(6.023*10^23) to get energy for one mole quanta of wavelength.
This is neede to break one mole og I-I bond.
Now, apply energy conservation , Kinetic energy=energy for one mole radiation -bond energy
and this extra energy is distributed over 2 moles of I2.in the form of kinetic energy. So we can calculate kinetic energy of one atom by dividing this value by 2 mole(i.e. 2*6.023*10^23)