Selection Rules and Why Some Transitions Never Appear · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Why some spectral lines never appear

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

  1. Why do nitrogen and oxygen show no infrared absorption?

    • They have no electrons at all
    • Stretching them never builds a dipole
    • They absorb only visible light instead

  2. What must happen during a vibration for infrared absorption to occur?

    • The dipole must change
    • The molecule must break apart
    • The temperature must rise first

  3. Selection rules decide which jumps appear in both emission and absorption spectra.

    Circle one:   True   False

  4. Nitrogen shows a Raman signal but no infrared band. Why?

    • Raman needs changing polarisability
    • Raman follows no rules at all
    • Infrared light cannot reach nitrogen molecules

  5. Why is carbon dioxide a greenhouse gas while nitrogen is not?

    • Carbon dioxide is heavier than nitrogen
    • Its dipole changes, nitrogen never does
    • Nitrogen reflects all infrared back to space

  6. Why is there a gap between the P and R branches?

    • The instrument cannot record the centre
    • The molecules stop rotating at the centre
    • Delta J = 0 is forbidden

  7. A sample glows long after the lamp turns off. A student calls it fast fluorescence. What is wrong?

    • Fluorescence is always invisible
    • Slow glow is phosphorescence
    • Afterglow breaks the conservation laws

  8. A faint band appears near twice the fundamental frequency. How do you classify it?

    • A forbidden transition that must be an error
    • Proof that the selection rules are wrong
    • A weak overtone, allowed but unlikely
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Answer key

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

Why some spectral lines never appear W1-mt_oJDr81W4fW-s1

  1. Stretching them never builds a dipole · Equal sharing means the dipole stays zero throughout the stretch.
  2. The dipole must change · No changing dipole means no infrared absorption at all.
  3. True · A forbidden jump stays missing from both kinds of spectrum.
  4. Raman needs changing polarisability · Different grips for different methods.
  5. Its dipole changes, nitrogen never does · The dipole requirement sorts greenhouse gases from silent ones.
  6. Delta J = 0 is forbidden · The missing line marks a forbidden step.
  7. Slow glow is phosphorescence · Slow light fingerprints a forbidden step.
  8. A weak overtone, allowed but unlikely · Weak is not forbidden. It breaks no rule, it just jumps two steps.
Worksheet · LightMySky