We have been considering all along, the behaviour of the MO, [eq (7.4)] which is called a bonding orbital. It is refered to as bonding because it increases the charge density between the nuclei (relative to separately incremented charge densities of and . Another MO, which does the opposite of this MO is called the antibonding MO as it decreases the electron density between the nuclei (in comparison to the non interacting or independently contributing AOs) and it is defined as
(7.4)
Just as in the case of the bonding MO , we can sketch the antibonding MO, in different ways. We shall show these sketches in fig 7.7. The contours of in the xy plane are shown in fig 7.7(a). It is seen that there are two sets of contours separated by a nodal plane. The value of the wavefunction in the plane bisecting the two nucleii (represented by the yZ plane containing x = 0) is identically zero. The contours on the right C1 , C2 , C3 and C4 ) have positive values and the contours on the left (C1 ', C2 ', C3 ' and C'4 ) have negative values. This is an antibonding orbital because the charge density between the nuclei is smaller than the charge density resulting from the sum of the two non-interacting charge densities, and .
A three dimensional plot showing as a function of points in the xy plane is shown in fig 7.7(b). It is seen that near one of the nuclei (the one on the right) has a maximum value (because of the maximum in which has a positive sign). Near the nucleus B, has a minimum value because appears with a negative sign in and has a large value near nucleus B. The density profile of is shown in fig 7.7(c)
Fig 7.7(a)The contours of the MO in a plane containing the internuclear axis A and B represent the nuclei. The values of on the contours C1, and C3 are 0.85 and 0.37 -3/2. The values on contours C1' and C3' are negative of the values on C1 and C3.
Fig 7.7 (b) A three dimensional plot of the MO
. The value of
on the upper peak is 1.1 -3/2 and on the lower peak, it is -1.1 -3/2.
and 
. Another MO, which does the opposite of this MO is called the antibonding MO as it decreases the electron density between the nuclei (in comparison to the non interacting or independently contributing AOs) and it is defined as 
, we can sketch the antibonding MO, 
in different ways. We shall show these sketches in fig 7.7. The contours of 
in the xy plane are shown in fig 7.7(a). It is seen that there are two sets of contours separated by a nodal plane. The value of the wavefunction 
in the plane bisecting the two nucleii (represented by the yZ plane containing x = 0) is identically zero. The contours on the right C1 , C2 , C3 and C4 ) have positive values and the contours on the left (C1 ', C2 ', C3 ' and C'4 ) have negative values. This is an antibonding orbital because the charge density between the nuclei is smaller than the charge density resulting from the sum of the two non-interacting charge densities, 
and 
. 
as a function of points in the xy plane is shown in fig 7.7(b). It is seen that near one of the nuclei (the one on the right) 
has a maximum value (because of the maximum in 
which has a positive sign). Near the nucleus B, 
has a minimum value because 
appears with a negative sign in 
and 
has a large value near nucleus B. The density profile of 
is shown in fig 7.7(c)
in a plane containing the internuclear axis A and B represent the nuclei. The values of 
-3/2. The values on contours C1' and C3' are negative of the values on C1 and C3. 
on the upper peak is 1.1 
