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Figure 1.1. Illustration of localization of electron density in sigma ( σ ) and pi ( π ) bonding between
two atoms. The σ bonding between two atoms is shown at the top while π and σ bonding between
two atoms is illustrated underneath.
molecule is lower than that of the separate atoms because interaction of the electron within
a single atom is much stronger when there is only one nucleus.
Let us now consider the second sigma orbital (2 σ ), shown in Eq. (1.4) . This wavefunc-
tion has symmetry identical to that of the 1 σ orbital, described in Eq. (1.3) . Schrödinger's
equation calculates that this wavefunction has a higher energy than the 1 σ orbital, and
also higher than either of the individual atomic orbitals. This can be explained in terms of
destructive interference of the two orbitals. The point in space which is equidistant from
each nuclei and intersects the internuclear axis has a wavefunction equal to zero. This is
known as the nodal plane and is referred to briefly in Section 1.2.2 . Both opposing orbit-
als cancel each other on this plane as result of destructive interference. This 2 σ bonding is
known as an antibonding orbital and is denoted as 2 σ *. This orbital excludes the electron
from the internuclear region and relocates it to outside the region of bonding. The net result
of this electron relocation is that the orbital is repulsive, pulling the nuclei apart. This is the
main reason why the antibonding molecular orbital 2 σ * exhibits higher energy than the
1 σ molecular orbital.
A molecular orbital exhibits “antibonding” properties when the electron density between
the two nuclei concerned is lower than would otherwise be predicted if there were no
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