13.3.4—Terms: planar when describing the arrangement

A planar arrangement places the stated atoms in the same geometric plane. In ethene, each carbon is sp² and the carbon atoms and attached substituents are approximately planar.

Planarity follows from the local orbital arrangement and restricted rotation around a C=C bond. It is a structural description, not a claim that every atom in a large molecule lies in one plane.

The two carbon atoms and four hydrogens of ethene form a planar group. Replacing the C=C by a single bond changes the rotational freedom and may remove that planar constraint.

Planar does not mean flat in every dimension of the whole molecule, and it does not by itself prove aromaticity.