46
ATOMIC STRUCTURE AND BONDING
To indicate resonance forms, we use a doubleheaded arrow between the contributing structures.
This arrow is reserved for resonance structures and
never used elsewhere. The difference between the
two structures is that the electrons in the π bonds
have been redistributed, and we can illustrate this by
use of another type of arrow, a curly arrow. This
arrow is used throughout chemistry to represent the
movement of two electrons. In the benzene case,
a cyclic movement of electrons accounts for the
apparent relocation of double bonds, though there
are two ways we might show this process; both are
equally satisfactory.
Benzene is a nice example to choose to illustrate
the concept of resonance. It is not the best example
for explaining the rules governing the use of curly
arrows, so we must move to some simpler compounds. Being able to draw curly arrows is an essential skill for an organic chemist, and you will see
from a cursory glance at the following chapters just
how frequently they are employed. We shall use the
same curly arrows and precisely the same principles
for predicting the outcome of chemical reactions (see
Section 5.1). They allow us to follow bond making
and bond breaking processes, and provide us with a
device we can use to keep track of the electrons.
• The curly arrow represents the movement of two
electrons.
• The tail of the arrow indicates where the electrons
are coming from, and the arrowhead where they
are going to.
• Curly arrows must start from an electron-rich
species. This can be a negative charge, a lone pair,
or a bond.
• Arrowheads must be directed towards an electrondeficient species. This can be a positive charge,
the positive end of a polarized bond, or a
suitable atom capable of accepting electrons, i.e.
an electronegative atom.
In our brief introduction to Lewis structures (see
Section 2.2), we paid particular attention to valency,
the number of bonds an atom could make to other
atoms via the sharing of electrons. We must now
broaden this idea to consider atoms in a molecule
that are no longer neutral, but which carry a formal
positive or negative charge. This means we are
considering cations and anions, as in ionic bonding,
but the atom involved is still part of a molecule,
and the molecule consequently also carries a formal
charge. We have already met a few such entities
in this chapter, e.g. the ammonium and hydronium
cations, looking specifically at the molecular orbital
descriptions (see Section 2.6.3). As indicated above,
the use of curly arrows may involve species with
positive or negative charges.
As simple examples, ammonia and water are neutral molecules. Nitrogen has five valence electrons,
and it acquires a stable octet of electrons in making
three bonds to hydrogen atoms. Each hydrogen has
its stable arrangement of two electrons. The nitrogen
in ammonia also carries a lone pair of electrons. Oxygen, with six valence electrons, makes two bonds to
hydrogen atoms. Its octet of electrons will carry two
lone pairs.
H N H
H
H O H
H N H
H
H O H
H
H
ammonia
water
ammonium
cation
hydronium
cation
We can deduce the charge associated with ammonia and water by simply considering that the component atoms are neutral, that all we have done is
share the electrons, so the molecules must also be
neutral. The formal charge on an individual atom
can be assessed more rigorously by subtracting the
number of valence electrons assigned to an atom in
its bonded state from the number of valence electrons it has as a neutral free atom. Electrons in bonds
are considered as shared equally between the atoms,
whereas unshared lone pairs are assigned to the atom
that possesses them.
formal
charge
=
number of valence
electrons as neutral
free atom
–
number of
valence electrons
assigned in
bonded state
Hence, for nitrogen, the number of valence electrons in a free atom is five. In ammonia, the number
of assigned electrons is also five (three in bonds plus
a lone pair). Therefore, the formal charge on nitrogen
is zero. For hydrogen, the formal charge is also zero,
since the number of valence electrons is one, and the
number of assigned electrons is one. For oxygen, the
number of valence electrons in a free atom is six. In
water, the number of assigned electrons is also six
ATOMIC STRUCTURE AND BONDING
To indicate resonance forms, we use a doubleheaded arrow between the contributing structures.
This arrow is reserved for resonance structures and
never used elsewhere. The difference between the
two structures is that the electrons in the π bonds
have been redistributed, and we can illustrate this by
use of another type of arrow, a curly arrow. This
arrow is used throughout chemistry to represent the
movement of two electrons. In the benzene case,
a cyclic movement of electrons accounts for the
apparent relocation of double bonds, though there
are two ways we might show this process; both are
equally satisfactory.
Benzene is a nice example to choose to illustrate
the concept of resonance. It is not the best example
for explaining the rules governing the use of curly
arrows, so we must move to some simpler compounds. Being able to draw curly arrows is an essential skill for an organic chemist, and you will see
from a cursory glance at the following chapters just
how frequently they are employed. We shall use the
same curly arrows and precisely the same principles
for predicting the outcome of chemical reactions (see
Section 5.1). They allow us to follow bond making
and bond breaking processes, and provide us with a
device we can use to keep track of the electrons.
• The curly arrow represents the movement of two
electrons.
• The tail of the arrow indicates where the electrons
are coming from, and the arrowhead where they
are going to.
• Curly arrows must start from an electron-rich
species. This can be a negative charge, a lone pair,
or a bond.
• Arrowheads must be directed towards an electrondeficient species. This can be a positive charge,
the positive end of a polarized bond, or a
suitable atom capable of accepting electrons, i.e.
an electronegative atom.
In our brief introduction to Lewis structures (see
Section 2.2), we paid particular attention to valency,
the number of bonds an atom could make to other
atoms via the sharing of electrons. We must now
broaden this idea to consider atoms in a molecule
that are no longer neutral, but which carry a formal
positive or negative charge. This means we are
considering cations and anions, as in ionic bonding,
but the atom involved is still part of a molecule,
and the molecule consequently also carries a formal
charge. We have already met a few such entities
in this chapter, e.g. the ammonium and hydronium
cations, looking specifically at the molecular orbital
descriptions (see Section 2.6.3). As indicated above,
the use of curly arrows may involve species with
positive or negative charges.
As simple examples, ammonia and water are neutral molecules. Nitrogen has five valence electrons,
and it acquires a stable octet of electrons in making
three bonds to hydrogen atoms. Each hydrogen has
its stable arrangement of two electrons. The nitrogen
in ammonia also carries a lone pair of electrons. Oxygen, with six valence electrons, makes two bonds to
hydrogen atoms. Its octet of electrons will carry two
lone pairs.
H N H
H
H O H
H N H
H
H O H
H
H
ammonia
water
ammonium
cation
hydronium
cation
We can deduce the charge associated with ammonia and water by simply considering that the component atoms are neutral, that all we have done is
share the electrons, so the molecules must also be
neutral. The formal charge on an individual atom
can be assessed more rigorously by subtracting the
number of valence electrons assigned to an atom in
its bonded state from the number of valence electrons it has as a neutral free atom. Electrons in bonds
are considered as shared equally between the atoms,
whereas unshared lone pairs are assigned to the atom
that possesses them.
formal
charge
=
number of valence
electrons as neutral
free atom
–
number of
valence electrons
assigned in
bonded state
Hence, for nitrogen, the number of valence electrons in a free atom is five. In ammonia, the number
of assigned electrons is also five (three in bonds plus
a lone pair). Therefore, the formal charge on nitrogen
is zero. For hydrogen, the formal charge is also zero,
since the number of valence electrons is one, and the
number of assigned electrons is one. For oxygen, the
number of valence electrons in a free atom is six. In
water, the number of assigned electrons is also six