En anglais

Why some spectral lines never appear

Science · Chemistry · ages 19-20

Name ______________________ Date ____________

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
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
Selection rules decide which jumps appear in both emission and absorption spectra.
Circle one: True False
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
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
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
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
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

Answer key

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

Stretching them never builds a dipole · Equal sharing means the dipole stays zero throughout the stretch.
The dipole must change · No changing dipole means no infrared absorption at all.
True · A selection rule applies to a pair of levels, so it governs emission and absorption alike.
Raman needs changing polarisability · Infrared needs a changing dipole, while Raman needs a changing polarisability.
Its dipole changes, nitrogen never does · The dipole requirement sorts greenhouse gases from silent ones.
Delta J = 0 is forbidden · The missing line marks a forbidden step.
Slow glow is phosphorescence · Slow light fingerprints a forbidden step.
A weak overtone, allowed but unlikely · Weak is not missing. Overtones break the simple one-step rule only because real bonds are not perfect springs, so they appear faintly.