The Scheme 2.2 shows the different energy trends for the first row biatomic
molecules labeled (a) and for those labeled (b) and indicates that for the molecules
(b) an energy inversion of r
þ
g and p u is active. This is connected to the small
energy difference between s and p orbitals of the interacting atoms.
On proceeding along the first row of the periodic table, increasing the atomic
number, the electronic population in Table 2.1 is expected, where the bond strength
of the molecules is associated to the number and the type of populated symmetry
orbitals.
2500
E(2p-2s)/(kJ mol
-1 )
(b)
(a)
Li
Be
B
C
N
O
F
2000
1500
1000
500
0
Scheme 2.2 Energy trend for the first row biatomic molecules as a function of the atomic species
Table 2.1 Orbital population
for the first row biatomic
molecules
Molecule/
ion
2r
þ
g
2r
þ
u
3r
þ
g
1p u
1p g
3r
þ
u
Li 2
2
Be 2
2
2
B 2
2
2
2
C 2
2
2
4
N 2
2
2
2
4
N 2
+
2
2
1
4
N 2
–
2
2
2
4
1
O 2
2
2
2
4
2
O 2
+
2
2
2
4
1
O 2
–
2
2
2
4
3
O 2
2–
2
2
2
4
4
F 2
2
2
2
4
4
Ne 2
2
2
2
4
4
2
28
2 Symmetry Orbitals for Different Bis and Polyatomic Molecules