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Energy of Photons

Electronic transitions between the different orbitals can cause a coloring of chemical substances. Light in the visible region (400 nm to 800 nm) possesses energies between 300 kJ mol−1 and 120 kJ mol−1. Because electrons are excited to a higher energy level during absorption of light, the energy difference (∆E) between the two energy levels has to correspond to the energy of the photons:

 ∆E = E2 E1 =                                                                             

The UV region of the electromagnetic spectrum comprises wavelengths between 200 nm and 400 nm. That equals an energy of 600 kJ mol−1 to 300 kJ mol−1. With this, photons in the UV and visible region (200 nm to 800 nm) have an energy of 600 kJ mol−1 to 120 kJ mol−1. Hence, the energy of photons can be larger than the enthalpies of single-bonds. For example, the enthalpies of the C–H bond equals to 413 kJ mol−1 and the enthalpy of the C–Cl bond equals to 339 kJ mol−1.


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