The Reflection Operation (σ)

The reflection operation can be pictured as follows: take each atom in the molecule and move it toward the reflection plane along a line perpendicular to that plane. Continue moving the atom through the plane to a point equidistant from the plane on the opposite side of the plane. If the resulting configuation is indistinguishable from the original, we say there exists a symmetry plane in the molecule.

Note the differences in the following examples:

  • The ammonia molecule contains three identical reflection planes. All are designated as vertical symmetry planes (σv) because they contain the principle rotation axis.
  • The water molecule contains two different reflection planes.
  • And benzene contains a total of seven reflection planes, one horizontal plane (σh) and six vertical planes (σv and σd).

Reflection planes: