A triple bond includes exactly one bond designated by this letter.
A quantity represented by this letter symbolizes substituent effects and multiplies rho in the Hammett equation. A German word is the origin of using this letter to represent molecular mirror planes. In collision theory, the sum of the atomic radii, or reaction cross-section, is symbolized by this letter. A chemical company named for this letter merged with (*)  Aldrich in 1975. Hydrogen's two frontier orbitals are both labelled by this letter. The head-on collision of either in-phase or out-of-phase orbitals creates molecular orbitals designated by this letter lying along the bonding axis. A triple bond includes exactly one bond designated by this letter. For 10 points, name this Greek letter that names the counterpart to pi bonds.
A quantity represented by this letter symbolizes substituent effects and multiplies rho in the Hammett equation.
igma  [or  sigma  bonds; or  sigma  orbitals]