Q.Compare the structures of H₂O and H₂O₂.

Step 1. Water (H₂O): three atoms -- one O, two H -- bonded as H-O-H, with a bond angle of 104.5°. Any three atoms are automatically coplanar (three points define a plane), so H₂O is inherently a planar, bent (V-shaped) molecule. Its oxygen is $sp^3$ hybridised (two bond pairs + two lone pairs).

Step 2. Hydrogen peroxide (H₂O₂): four atoms -- two O, two H -- bonded as H-O-O-H. Both oxygens are likewise $sp^3$ hybridised (each with one O-H bond, one O-O bond, and two lone pairs).

Step 3. Unlike H₂O, H₂O₂ has enough atoms (four) that they need NOT be coplanar -- and in fact they are not: repulsion between the lone pairs on the two oxygens twists the two O-H bonds out of a single plane, giving a non-planar, skewed "open-book" shape, with the two hydrogens on the "pages" and the O-O bond as the "spine".

Step 4. This skew is measured by the dihedral angle between the two O-H planes: 111.5° in the gas phase, closing to 90.2° in the solid phase (where hydrogen bonding between neighbouring H₂O₂ molecules pulls the "book" more shut); correspondingly, the O-O-H bond angle opens from 94.8° (gas) to 101.9° (solid). This makes H₂O₂ the smallest known molecule to show hindered rotation about a single bond (here, the O-O bond) -- a structural feature water, with only one central bond-free atom, cannot show at all. …

Contrast the two structures atom-by-atom: H2O's three atoms are inherently planar/bent; H2O2's four atoms allow (and lone-pair repulsion causes) a genuinely non-p …

• Assuming both molecules must be planar because their oxygens are both sp3 -- hybridisation state does not by itself force planarity; H2O2 has enough atoms, and enough lone-pair repulsion, to twist out o …