Chapter U1.17, Problem 6E

Interpretation Introduction

Interpretation:

The evidence for the blue-green color obtained during the flame test of copper salt needs to be explained.

Concept introduction:

When salt is heated in burner, electron from valence shell gets excited and moves from ground state to excited state. This electron jumps back to the ground state and imparts flame color.

Answer to Problem 6E

Copper salt absorbs light in red zone. So, the expected emission zone is blue-green.

Explanation of Solution

Emission wavelength of a substance is complementary to absorption wavelength. It means if a substance absorbs longer wavelength then it will emit light of shorter wavelength. The complimentary colors can be obtained from Newton’s color wheel which is represented as follows:

As blue-green color is emitted by copper salt it means copper absorbs red light. Wavelength of red light is very high (low energy) which means copper needs very low energy for ionization.

This is expected from the electronic configuration of Cu.

$\text{Cu }\left(29\right)=\left[Ar\right]4s^{1}3d^{10}$

Here, [Ar] stands for the electronic configuration of argon gas which is an inert gas.

Once the electron is removed from the 4s sub-shell then remaining electrons have fully filled 3d sub-shell.

So, copper requires very low energy in the first ionization.

Conclusion

Copper absorbs red light and emits blue-green light. Ionization energy of copper is less which is supplied by red light (a light with greater wavelength and lower energy).