Schematic, qualitatively correct π molecular orbitals can be easily generated from a linear array of n p atomic orbitals by following a few simple rules.
 The lowestenergy MO always has all p orbitals in phase, making it symmetric with respect to endforend reflection:
This MO has no "vertical" nodes, that is, points along the chain at which neighboring porbitals are out of phase.
The next MO has a single vertical node, and one half of the MO is the "antimirror image" of the other half; that is, the MO is antisymmetric with respect to endforend reflection. An example is the π* MO of ethylene, shown above.
The third MO has two vertical nodes, and is symmetric. To find the position of the vertical nodes, remember that half of the porbitals need to reflect the other half. An example is the third MO of butadiene, shown above.
 Molecular orbitals continue to add vertical nodes and alternate symmetric/antisymmetric until the highestenergy MO is reached, which has a vertical node between each atom. When combining n p orbitals, the highestenergy π MO will have n1 vertical nodes.
Some examples of π MO diagrams are linked below. "S" is "symmetric", "A" is "antisymmetric" to endforend reflection (useful for thinking about disrotatory pericylic processes). Symmetries with respect to 180° rotation (useful for thinking about conrotatory processes) are the opposite.
