atoms sharing electrons
as in a single bond
(that is, a sigma bond)
may also share electrons
in an orbital
with a node
passing through the two atoms.
This adds a second, weaker bonding interaction (a pi bond); the combination is termed a double bond. A twisting motion that forces the nodal plane at
one atom to become perpendicular to the nodal plane on the
eliminates the (signed) overlap
between the atomic orbitals, destroying the pi bond. The energy
required to do this creates a large barrier to rotation about the bond (see triple bond).