Advanced level inorganic chemistry: UV and visible light absorption spectroscopy - halogen molecules

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Doc Brown's Advanced Chemistry: PART 15.5 uv and visible light absorption spectroscopy - halogens

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15.5.1 The origin of colour, the wavelengths of visible light, our perception!

15.5.2 uv-visible spectroscopy theory, spectrometer, examples of absorption & reflectance spectra explained

15.5.3 uv-visible absorption spectra - index of examples: uses, applications, more on the chemistry of colour

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

absorption spectra and the the colours of halogen molecules

uv-absorption spectra of the halogens spectrum of chlorine bromine iodine group 7 group 17 molecules doc brown's advanced level chemistry 

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:

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

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

and λmax  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 λmax of <330 nm in the uv, a 'paler' colour,

astatine (At2) is a very dark coloured solid/vapour with an expected  λmax 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 ∆Eelec 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.



Key words & phrases: interpreting the uv-visible absorption spectrum of chlorine, identifying the maximum absorption peaks in the uv-visible absorption spectrum of chlorine, explaining the uv-visible absorption spectrum of chlorine, how to use the visible absorption spectra of chlorine to explain the colour of chlorine, applications of the uv-visible absorption spectrum of chlorine Key words & phrases: interpreting the uv-visible absorption spectrum of bromine, identifying the maximum absorption peaks in the uv-visible absorption spectrum of bromine, explaining the uv-visible absorption spectrum of bromine, how to use the visible absorption spectra of bromine to explain the colour of bromine, applications of the uv-visible absorption spectrum of bromine Key words & phrases: interpreting the uv-visible absorption spectrum of iodine, identifying the maximum absorption peaks in the uv-visible absorption spectrum of iodine, explaining the uv-visible absorption spectrum of iodine, how to use the visible absorption spectra of iodine to explain the colour of iodine, applications of the uv-visible absorption spectrum of iodine Key words & phrases: interpreting the uv-visible absorption spectrum of fluorine astatine, identifying the maximum absorption peaks in the uv-visible absorption spectrum of fluorine astatine, explaining the uv-visible absorption spectrum of fluorine astatine, how to use the visible absorption spectra of fluorine astatine to explain the colour of fluorine astatine, applications of the uv-visible absorption spectrum of fluorine astatine interpreting the uv-visible absorption spectra of group 7/17 halogens, identifying the maximum absorption peaks in the uv-visible absorption spectra of group 7/17 halogens, explaining the uv-visible absorption spectra of group 7/17 halogens, how to use the visible absorption spectra of group 7/17 halogens to explain the different colours of group 7/17 halogens, applications of the uv-visible absorption spectra of group 7/17 halogens explaining the colour color of iodine in different solvents


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UV and visible spectroscopy index

SPECTROSCOPY INDEXES

Index of Advanced A Level Notes on the Halogens

All Advanced A Level Inorganic Chemistry Notes

GCSE/IGCSE Revision Notes on the Halogens

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