Electronic Spectra, Franck-Condon and the Origin of Colour · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Why molecules wear colour

Science · Chemistry · ages 19-20
Name ______________________   Date ____________

  1. Why do molecules show broad bands where atoms show sharp lines?

    • Molecules are always hotter than atoms
    • Each jump bundles many sub jumps
    • Molecules contain no electrons

  2. What does the Franck Condon principle say about an electronic jump?

    • Nuclei stand still, so it is vertical
    • The nuclei move first, then the electron follows
    • Electrons and nuclei always move together

  3. Longer conjugation moves absorption toward longer wavelength.

    Circle one:   True   False

  4. What sets the brightest peak inside an electronic band?

    • The temperature of the lamp
    • The number of molecules in the room
    • The vibrational overlap

  5. Two dyes differ by one double bond. Which absorbs at longer wavelength, and why?

    • The shorter one, because small gaps cost more
    • More conjugated, so gap is narrower
    • Both absorb identically whatever the structure

  6. A carbonyl compound shows a strong deep ultraviolet band and a weak one nearer the visible. How do you assign them?

    • Strong is n to pi*, weak is pi to pi*
    • Strong is pi to pi*, weak is n to pi*
    • Both bands are the same transition

  7. A metal complex changes from a weak field ligand to a strong field one. What happens to its colour?

    • Absorption moves blue as split widens
    • Absorption shifts toward the red as the split widens
    • The colour cannot change whatever the ligand

  8. A student draws the electronic jump as a diagonal arrow with nuclei shifting mid leap. What is wrong?

    • Nothing, diagonal arrows are standard
    • Arrows are never drawn between curves
    • Vertical, since nuclei stand still
LightMySky · lightmysky.comW1-mt_y1hKy9nvAY-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Why molecules wear colour W1-mt_y1hKy9nvAY-s1

  1. Each jump bundles many sub jumps · One line spreads into a band system.
  2. Nuclei stand still, so it is vertical · Electrons rearrange thousands of times faster than nuclei.
  3. True · A narrower gap costs a redder photon.
  4. The vibrational overlap · The vertical landing with the biggest overlap wins.
  5. More conjugated, so gap is narrower · Extra conjugation squeezes the gap toward the visible.
  6. Strong is pi to pi*, weak is n to pi* · Allowed jumps shine, symmetry blocked ones whisper.
  7. Absorption moves blue as split widens · A wider gap drinks bluer light.
  8. Vertical, since nuclei stand still · The leap is too fast for the nuclei to follow.
Worksheet · LightMySky