34
ATOMIC STRUCTURE AND BONDING
σ bonds
The simplest compounds to consider here are ammonia and water. It is apparent from the above electronic configurations that nitrogen will be able to
bond to three hydrogen atoms, whereas oxygen can
only bond to two. Both compounds share part of the
tetrahedral shape we saw with sp
3 -hybridized carbon. Those orbitals not involved in bonding already
have their full complement of electrons, and these
occupy the remaining part of the tetrahedral array
(Figure 2.21). These electrons are not inert, but play
a major role in chemical reactions; we refer to them
as lone pair electrons.
These orbital pictures tend to get a little confusing,
in that we really need to put in the elemental symbol
to distinguish it from carbon, and we usually wish
to show the lone pair electrons. We accordingly
use a compromise representation that employs the
cleaner line drawings for part of the structure and
shows the all-important orbital with its lone pair
of electrons. These are duly shown for ammonia
and water.
The tetrahedral geometry resultant from these sp
3 -
hybridized nitrogen and oxygen atoms is found to
exist in both ammonia and water. Bond angles in
these molecules are not quite the 109
◦ of the perfect
tetrahedron, because the electrons in the lone pair
atomic orbital are not involved in bonding. They are,
therefore, closer to the nucleus than the electrons
in the N–H or O–H bond σ molecular orbitals.
Lone pairs thus tend to exert a greater electronic
repulsive force between themselves, and also towards
the bonding electrons, than the σ bonding electrons
do to each other. The net result is that bond angles
between lone pairs, or between lone pairs and σ
bonds, are somewhat greater than between σ bonds,
a distortion of the perfect tetrahedral array.
Lone pair electrons may be used in bonding. Since
they already have a complement of two electrons,
bonds will need to be made to an atom that is electron
deficient, e.g. a proton. Thus, the ammonium cation
and the hydronium cation also share tetrahedral
geometry, and each possesses a σ bond formed from
lone pair electrons.
+ H
+
N
H
H
H
N
H
H
H
N H
H
H
H
=
O
H
H
+ H
+
O
H
H
O H
H
H
=
ammonium
cation
hydronium
cation
The hydronium cation still possesses a lone pair of
electrons. It does not bond to a second proton for the
simple reason that the cation would then be required
to take on an unfavourable double positive charge.
π bonds
When we consider double bonds to oxygen, as in
carbonyl groups (C=O) or to nitrogen, as in imine
functions (C=N), we find that experimental data are
best accommodated by the premise that these atoms
are sp
2 hybridized (Figure 2.22). This effectively
follows the pattern for carbon–carbon double bonds
(see Section 2.6.2). The double bond is again a
combination of a σ bond plus a π bond resulting
from overlap of p atomic orbitals. The carbonyl
formation of three
σ bonds in ammonia
with one lone pair
formation of two
σ bonds in water with
two lone pairs
N
H
H
H
O
H
H
107˚
110˚
105˚
Figure 2.21 sp
3 hybrid orbitals: ammonia and water
ATOMIC STRUCTURE AND BONDING
σ bonds
The simplest compounds to consider here are ammonia and water. It is apparent from the above electronic configurations that nitrogen will be able to
bond to three hydrogen atoms, whereas oxygen can
only bond to two. Both compounds share part of the
tetrahedral shape we saw with sp
3 -hybridized carbon. Those orbitals not involved in bonding already
have their full complement of electrons, and these
occupy the remaining part of the tetrahedral array
(Figure 2.21). These electrons are not inert, but play
a major role in chemical reactions; we refer to them
as lone pair electrons.
These orbital pictures tend to get a little confusing,
in that we really need to put in the elemental symbol
to distinguish it from carbon, and we usually wish
to show the lone pair electrons. We accordingly
use a compromise representation that employs the
cleaner line drawings for part of the structure and
shows the all-important orbital with its lone pair
of electrons. These are duly shown for ammonia
and water.
The tetrahedral geometry resultant from these sp
3 -
hybridized nitrogen and oxygen atoms is found to
exist in both ammonia and water. Bond angles in
these molecules are not quite the 109
◦ of the perfect
tetrahedron, because the electrons in the lone pair
atomic orbital are not involved in bonding. They are,
therefore, closer to the nucleus than the electrons
in the N–H or O–H bond σ molecular orbitals.
Lone pairs thus tend to exert a greater electronic
repulsive force between themselves, and also towards
the bonding electrons, than the σ bonding electrons
do to each other. The net result is that bond angles
between lone pairs, or between lone pairs and σ
bonds, are somewhat greater than between σ bonds,
a distortion of the perfect tetrahedral array.
Lone pair electrons may be used in bonding. Since
they already have a complement of two electrons,
bonds will need to be made to an atom that is electron
deficient, e.g. a proton. Thus, the ammonium cation
and the hydronium cation also share tetrahedral
geometry, and each possesses a σ bond formed from
lone pair electrons.
+ H
+
N
H
H
H
N
H
H
H
N H
H
H
H
=
O
H
H
+ H
+
O
H
H
O H
H
H
=
ammonium
cation
hydronium
cation
The hydronium cation still possesses a lone pair of
electrons. It does not bond to a second proton for the
simple reason that the cation would then be required
to take on an unfavourable double positive charge.
π bonds
When we consider double bonds to oxygen, as in
carbonyl groups (C=O) or to nitrogen, as in imine
functions (C=N), we find that experimental data are
best accommodated by the premise that these atoms
are sp
2 hybridized (Figure 2.22). This effectively
follows the pattern for carbon–carbon double bonds
(see Section 2.6.2). The double bond is again a
combination of a σ bond plus a π bond resulting
from overlap of p atomic orbitals. The carbonyl
formation of three
σ bonds in ammonia
with one lone pair
formation of two
σ bonds in water with
two lone pairs
N
H
H
H
O
H
H
107˚
110˚
105˚
Figure 2.21 sp
3 hybrid orbitals: ammonia and water