Chemistry

Problem

Three photons have energies of

0.03\,\text{eV}

(Photon X),

0.2\,\text{eV}

(Photon Y), and

3\,\text{eV}

(Photon Z). Which row correctly matches each photon to the type of molecular transition it can most readily cause? a)

X

—vibrational;

Y

—rotational;

Z

—electronic b)

X

—rotational;

Y

—vibrational;

Z

—electronic c)

X

—rotational;

Y

—electronic;

Z

—vibrational d)

X

—electronic;

Y

—vibrational;

Z

—rotational

Intuitive explanation

The key insight is that different photon energies correspond to different regions of the electromagnetic spectrum, and each region excites different types of molecular motion. Low-energy photons (infrared) cause vibrations, mid-range photons (visible/UV) cause electronic transitions, and very low-energy photons (microwave/far-infrared) cause rotations. Match each photon's energy to the molecular process it's most likely to trigger by thinking about the typical energy scales: rotational transitions ( 0.0001–0.01 eV), vibrational transitions ( 0.01–0.5 eV), and electronic transitions ( 1–10 eV).

Intuitive explanation

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