The vibration of the carbon dioxide molecule is composed of four normal modes. Two modes are stretching the molecule along the covalently bound C and O. The other two are bending the linear molecule, basically changing the angle α(O-C-O). The bending modes are degenerate (they have the same vibrational frequency), which makes them indistinguishable in usual experiments. Thus, we only speak of one bending mode ν₂(E_1u). The stretching mode ν₃(A_1u) and the bending mode ν₂(E₁) are anti-symmetric with respect to the inversion center, while the stretching mode ν₁(A_1g) is symmetric. Carbon dioxide is in the D_∞h point group with the symmetry elements: S₂ and C_∞ axes (black), infinite σ_v planes (cyan), infinite C₂ axes, inversion center. [check symotter-gallery]