Describe the structure of hydrogen peroxide $(H_2O_2)$.

Hydrogen peroxide has a non-planar structure.
$a$. In the gas phase, the $H_2O_2$ molecule has a dihedral angle of $111.5^{\circ}$. The $O-O$ bond length is $147.5 \text{ pm}$ and the $O-H$ bond length is $95.0 \text{ pm}$.
$b$. In the solid phase at $110 \text{ K}$, the $H_2O_2$ molecule has a dihedral angle of $90.2^{\circ}$. The $O-O$ bond length is $145.8 \text{ pm}$ and the $O-H$ bond length is $98.8 \text{ pm}$.