Types of molecular orbitals

In a diatomic molecule, every molecular orbital formed from combining atomic orbitals falls into one of two symmetry classes: a sigma (σ) molecular orbital, which is symmetrical all the way around the internuclear (bond) axis, or a pi (π) molecular orbital, which is NOT symmetrical about that axis. Combining two 's' atomic orbitals — which are spherically symmetric in every direction to begin with — always produces σ-type molecular orbitals: one bonding σ (from adding the wave functions) and one antibonding σ* (from subtracting them), the antibonding one carrying a node — a plane of zero electron density — positioned exactly between the two nuclei. Taking the internuclear axis as z, combining two $p_z$ orbitals END-ON similarly gives a bonding σ(2pz) and an antibonding σ*(2pz) molecular orbital, by the same logic. The $p_x$ and $p_y$ orbitals, however, are NOT symmetric about the bond axis at all — each has a positive lobe on one side of the axis and a negative lobe on the other — so combining a PAIR of $p_x$ (or $p_y$) orbitals SIDEWAYS instead produces π bonding and π* antibonding molecular orbitals, whose electron density sits concentrate …