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Report Thread starter 1 year ago
There is a 6 marker that I need help with.
1) Colour is often used in chemistry to identify substances

Compare and contrast the origin of the colour of a copper(II) complex
with the origin of the colour of the copper(II) ion in a flame test.

You do not need to state any specific colours.
Badges: 6
Report 1 year ago
Well, the main difference is that in a flame test you observe the emitted light while in complex ions you perceive them as coloured due to absorption of some wavelengths.

Energy from the flame causes the electrons in an atom to become excited and when coming back to the ground state they release a quantum of energy in a form of a photon (the electron undergoes transition to a lower energy level). Due to electrons being able to occupy only certain energy levels, energies, and therefore wavelengths, of emitted photons are exactly determined (on the spectrum they are shown as 'fringes'. The emission spectrum is therefore unique for each element and flame test can precisely confirm the element tested.

In complex ions the colour observed is a mixture of wavelengths lacking the ones that were absorbed by the compound. Due to unoccupied energy levels (d orbitals) in transition metals' ions (including copper(II) ions) electrons can absorb photons from the electromagnetic spectrum, including the visible light and we can observe the complementary colour eg. copper ions absorb the light from red end of the visible spectrum of the light so we observe them as a light blue. Wavelengths absorbed depend on the structure of the complex (central atom, charge, ligands, shape/coordination number etc.).

You can refer the above to the copper ion ( Cu^{2+} has 9 d electrons only, and the orbital's energy are further differentiated by ligand molecules such as water).

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