The most stringent and most helpful
rule is the third one, which demands the same symmetry of orbitals
around the bond axis, or the line connecting atoms. The logic behind
this is shown above. Two atomic s orbitals have total symmetry about
the bond axis, and can be combined into a s-
MO as shown at (a). Two p orbitals oriented as in (b) have what
is termed p (pi) symmetry about the
bond axis. If you rotate them 180' around this axis, the electron
density will look the same, but the signs of the wave functions
in all of the density lobes will be reversed.
Each atomic p orbital has one positive
lobe and one negative lobe, and there is a surface plane halfway
between on which the electron probability falls to zero. Such a
zero- probability surface in a wave function is called a node. The
resulting p MO also has a positive lobe,
a negative lobe, and a zero-probability node halfway between.