As you saw in the spectroscopy lab, copper (II) ions have a Lambda_max of around

Question

As you saw in the spectroscopy lab, copper (II) ions have a Lambda_max of around 800nm. Colors like this are common for transition metals and their compounds, but a number of organic molecules also can absorb and reflect energy in the UV/visible spectrum, and therefore appear colored. In both cases, the colors absorbed are based on the electronic transitions of the highest-energy electrons. Which electrons in the copper ion have the highest energy? What about in the molecule shown below If the Lambda_max of the molecule below is around 250nm, what is the approximate separation between energy levels for that transition? What about for the copper ion?

Explanation / Answer

3d electrons of copper have the highest energy as it has electrons first fill up 4s orbital and then the 3d orbital.

The organic molecule is a conjugated system having extended double bond conjugation. electron in pi bonding orbitals have highest energy and colour is due to pi-pi* transition.

Maximum wavelength of absorption = 250 nm

Since, E = hc/ wavelength where h is Planck's constant and c = speed of light

therefore, separation between energy levels = 6.626 x10-34 Js x 3 x 108 ms-1 / 250 x 10-9 m

= 0.08 x 10-17 J

For Cu ions, maximum wavelength is at 800 nm.

Following same equation, E = 6.626 x10-34 Js x 3 x 108 ms-1 / 800 x 10-9 m = 0.025 x 10-17 J

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