The uv and visible absorption spectra of the Group 7/17 Halogens - the element diatomic molecules

As the Group 7/17 halogen molecule increases in size due successive additions a filled principal quantum level, the outer electron, both bonding and non-bonding, are less strongly held.

Therefore, for electron excitation, ∆E = hv, the energy needed decreases down the group as the molecule gets bigger in terms of filled principal quantum levels.

You can see this shift in the spectrum peak in the simplified diagram above, where down the group the absorption shifts towards the red end of the visible spectrum, producing an increasingly 'darker' colour..

Maximum absorbance wavelengths for the uv-visible absorption spectra of halogens:

λ_max  is 330 nm for chlorine (Cl₂, green gas/solution),

λ_max  is 420 nm for bromine (Br₂, dark red-orange vapour/liquid/solution)

and λ_max  is 520 nm for iodine (I₂, purple vapour/very dark solid, colour solution varies with solvent - see extra notes below).

Spectral data from https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.1929.0141 - a 1929 research paper from the (maybe the same?) laboratories of Liverpool University where I did my PhD from 1969 to 1973!)

Periodic table predictions for the absorption spectra of halogens

I couldn't find data for extremely reactive fluorine or radioactive astatine, so predictions:

fluorine (F₂) is a very pale yellow gas with an expected λ_max of <330 nm in the uv, a 'paler' colour,

astatine (At₂) is a very dark coloured solid/vapour with an expected  λ_max of >420 nm, still in the visible region and a much darker colour,

 so, another example of a periodic table group trend.

The colour of iodine in different media

Different solvents can have a small effect on the electronic energy levels of iodine molecules (outer electrons) and hence an effect on the ∆E_elec energy needed for molecular excitation from absorbing visible light photons.

(i) Non-polar or weakly polar solvents

In solvents like hexane, tetrachloromethane or trichloromethane, the iodine solution is a bright purple colour, similar to the vapour.

(ii) Highly polar solvents.

Iodine is a dark brown colour in water or ethanol, highly polar solvent molecules.

(iii) Aqueous potassium iodide solution.

When dissolved in potassium iodide there is an equilibrium between iodine molecules, iodide ions and triiodide ions.

I₂(aq)  +  I^-(aq)    I₃^-(aq)

Which means some of the colour is due to absorption by the I₃^- ion. as well as absorption of iodine molecules in a highly polar solvent. The iodide ion is colourless, doesn't absorb in the visible region.

(iv) Starch solution

The iodine molecules become encapsulated in the coiled polymer molecules of starch.

This affects the electronic energy levels of iodine to give a dark blue-black colour.

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