Methyl radical, symmetry axis

The methyl radical, CH3, with a planar trigonal structure, looks the same if it is rotated by 120, 240, or 360° about an axis (C3 axis in Fig. 5.18) perpendicular to the plane containing three equivalent carbon atoms. An operation that leaves a molecule (object) looking the same (or sending into itself or a position indistinguishable from the original) is a symmetry operation. Symmetry operations include seven symmetry elements in Table 5.6 which are commonly possessed by molecular systems. 

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