= Solution
A <diatomic molecule> has $3N-5=1$ vibrational <normal mode>: the bond stretch. For <hydrogen chloride>, opposite longitudinal displacements change the <electric dipole moment>, so the mode has <vibrational infrared activity>. The following original sketches show one phase of this motion and the three water modes; arrow lengths are schematic, not mass-weighted amplitudes.
\Image[/past-exam-of-the-chemistry-a-course-of-the-university-of-cambridge/2025/paper-1-vibrations.png]
{title=Original sketches of the <hydrogen chloride> stretch and the three <molecular vibrations> of water}
Back to article page
