Chemistry Reference
In-Depth Information
2
Atomic structure and bonding
2.1 Atomic structure
notation.
Ionic bonds
have been shown as a simple
electrostatic interaction of appropriate counter ions,
so that sodium chloride and magnesium chloride
are conveniently drawn as Na
+
Cl
−
and Mg
2
+
2Cl
−
respectively. It becomes increasingly difficult to
remove successive electrons from an atom, and ionic
bonding is not usually encountered for some atoms,
especially carbon. Organic chemistry, the study of
carbon compounds, is dominated by covalent bonding
and the sharing of electrons.
A
covalent bond
between atoms involves the
sharing of two electrons, one from each atom. The
sharing of two electrons is described as a
single
bond
, and is indicated in shorthand notation by a
single line. Depending upon the number of electrons
an atom carries, it is able to form a certain number
of bonds, and this number is called the
valency
of
the atom. The valency of hydrogen is 1, of oxygen 2,
of nitrogen 3, and carbon 4. This means that we can
indicate the bonding in simple organic molecules such
as methane, methanol, and methylamine via single
bonds (see Section 1.1).
Atoms
are composed of protons, neutrons and
electrons.
Protons
are positively charged,
electrons
carry a negative charge, and
neutrons
are uncharged.
In a neutral atom, the nucleus of protons and neutrons
is surrounded by electrons, the number of which is
equal to the number of protons. This number is also
thesameasthe
atomic number
of the atom. If the
number of electrons and protons is not equal, the
atom or molecule containing the atom will necessarily
carry a charge, and is called an
ion
. A negatively
charged atom or molecule is termed an
anion
,anda
positively charged species is called a
cation
.
The inert or
noble gases
, such as helium, neon,
and argon, are particularly unreactive, and this has
been related to the characteristic number of electrons
they contain, 2 for helium, 10 for neon
(
2
+
8
)
,
and 18 for argon
(
2
8
)
. They are described
as possessing 'filled shells' of electrons, which,
except for helium, contain eight electrons, an
octet
.
Acquiring a noble gas-like complement of electrons
governs the bonding together of atoms to produce
molecules. This is achieved by losing electrons, by
gaining electrons, or by sharing electrons associated
with the unfilled shell, and leads to what we term
ionic bonds or covalent bonds. The unfilled shell
involved in bonding is termed the
valence shell
,and
the electrons in it are termed
valence electrons
.
+
8
+
single bonds
H
C
H
C
H
C
H
H
O
H
H
H
H
N
H
H
H
H
methane
methanol
methylamine
Carbon is particularly versatile, in that it can
sometimes share two of its electrons with a second
carbon, with nitrogen, or with oxygen. It can even
use three of its four valencies in bonding to another
carbon, or to nitrogen. In this way, we generate
2.2 Bonding and valency
For many years now, these types of bonding have
been
represented
in
chemistry
via
a
shorthand
