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An absorbance spectrum is a graph that shows how much light is absorbed by the material at each wavelength of light. Both spectra are due to electronic transition, but for gaseous atoms, this spectrum is made up of lines or discontinuities, and for complex ions, it produces broad absorption spectrums/bands (calibration curve).

Why does this happen?

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  • $\begingroup$ Before asking, you should review the targeted search results like site:stackexchange.com OR site:libretexts.org OR site:wikipedia.org atomic versus molecular spectra $\endgroup$
    – Poutnik
    Commented May 1 at 6:17
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    $\begingroup$ Note atomic spectra only tend towards discrete lines at low pressure and temperature, see en.wikipedia.org/wiki/Spectral_line#Line_broadening_and_shift for some reasons why. Thinking about this may give you ideas for some of the reasons why more complex entities tend to show broader "lines". $\endgroup$
    – Ian Bush
    Commented May 1 at 6:18
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    $\begingroup$ Yeah. I did do a bit of research online, and understood that the surrounding ligands have both rotational and vibrational energy. Because the ligands can absorb any excess energy as increased vibrational and rotational energy, the central ion can now accept a wider range of frequencies. Isolated gaseous ions on the other hand will only absorb energy of the precise wavelength needed to transfer an electron from a lower energy to a higher energy 'level'. Could this be it? $\endgroup$
    – Ronith
    Commented May 1 at 7:07
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    $\begingroup$ Complex molecules have very many vibrational and rotational transitions and their spectra become crowded because of this, even in the gas phase. Additionally in solution (or in high pressure gas) the solvent interacts with the molecule shifting energy levels up and down just a little and so this leads to more crowding and so v wide spectral features. $\endgroup$
    – porphyrin
    Commented May 1 at 7:07
  • $\begingroup$ @porphyrin thanks for confirming. It makes sense to me now! $\endgroup$
    – Ronith
    Commented May 1 at 7:08

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