Past exam of the chemistry A course of the University of Cambridge / 2025 / Paper 1 / A4 / b / i / Solution

With $z$ along C=O and the molecule in the $xz$ plane, formaldehyde has $E$, $C_2(z)$, ${\sigma }_{xz}$ and ${\sigma }_{yz}$. It belongs to the C2v point group, $\boxed{C_{2v}}$. Keeping this axis convention is essential when assigning the labels $B_1$ and $B_2$.