C hapter 4 Material Classes, structure, and properties
94
One other type of chemical bonding is covalent bonding. In this
type of bonding, there is a high probability of electrons spending
most of their time in the region between atoms. Thus, for example,
when two atoms of hydrogen form a molecule, the shared electrons spend most of their time in between the two hydrogen atoms.
What would happen to the bonding if, for some reason, both
electrons were on one side? The bonding would no longer be
covalent. It would become an ionic bond because one atom would
have an excess of electrons, whereas the other one would have a
depletion of electrons. In general, although covalent bonds are
very strong, materials bonded in this manner have poor ductility
and poor electrical conductivity. Thus, for example, when silicon is
bent, bonds must be broken for deformation to occur. This is not
easy. In addition, for electrons to be mobile, bonds must be again
broken. Therefore, covalent bonded materials are usually brittle
and insulators.
Other materials are only partially covalent. Compounds, in
general, are neither purely ionic nor purely covalent. Take, for
example, sugar, which is composed of many units of the monomer
C 12 H 22 O 11 . In sugar, the atoms are tightly held by covalent bonds.
However, between the various C 12 H 22 O 11 units, there are no
covalent bonds and the interaction is weak. As a result, the crystals of sugar are easy to break. For a compound, the degree of
either ionic or covalent bonding depends on the position of the
elements in the periodic table. The greater the difference in electronegativity between the elements, the more ionic the bond, whereas
the smaller the difference, the greater the degree of covalency.
Another important type of bonding is metallic bonding. We define
that term as the ease with which atoms of elements lose valence
electrons; it provides a basis for classifying atoms into two general
groups, namely metals and nonmetals. In general, an element
that has one, two, or three valence electrons in to its outermost
shell will tend to lose the electrons and thus is considered a metal.
What, then, is particular about metallic bonding? In fact, it is the
idea that a metal is an aggregate of positively charged cores surrounded by a “sea” of electrons (see Figure 4.8). In the metallic
structure, the electrons actually hold the ions in place; otherwise
they would attract each other. One can think of this cohesion as a
mass of solid balls bonded by a very strong liquid glue. As a result of
this metallic structure, the path of an electron in the metallic bond
is completely random around the aggregate of ions (see Figure 4.9).
Therefore, metals are of course good conductors because electrons
can easily move in any direction. Second, metals usually have a high
Figure 4.8
A metallic bond forms when atoms give up their
valence electrons, which then form an electron
sea.
Core
Figure 4.9
The path of an electron in a metallic structure.
+
+
+
+
+
+
+
+
+
Electron
94
One other type of chemical bonding is covalent bonding. In this
type of bonding, there is a high probability of electrons spending
most of their time in the region between atoms. Thus, for example,
when two atoms of hydrogen form a molecule, the shared electrons spend most of their time in between the two hydrogen atoms.
What would happen to the bonding if, for some reason, both
electrons were on one side? The bonding would no longer be
covalent. It would become an ionic bond because one atom would
have an excess of electrons, whereas the other one would have a
depletion of electrons. In general, although covalent bonds are
very strong, materials bonded in this manner have poor ductility
and poor electrical conductivity. Thus, for example, when silicon is
bent, bonds must be broken for deformation to occur. This is not
easy. In addition, for electrons to be mobile, bonds must be again
broken. Therefore, covalent bonded materials are usually brittle
and insulators.
Other materials are only partially covalent. Compounds, in
general, are neither purely ionic nor purely covalent. Take, for
example, sugar, which is composed of many units of the monomer
C 12 H 22 O 11 . In sugar, the atoms are tightly held by covalent bonds.
However, between the various C 12 H 22 O 11 units, there are no
covalent bonds and the interaction is weak. As a result, the crystals of sugar are easy to break. For a compound, the degree of
either ionic or covalent bonding depends on the position of the
elements in the periodic table. The greater the difference in electronegativity between the elements, the more ionic the bond, whereas
the smaller the difference, the greater the degree of covalency.
Another important type of bonding is metallic bonding. We define
that term as the ease with which atoms of elements lose valence
electrons; it provides a basis for classifying atoms into two general
groups, namely metals and nonmetals. In general, an element
that has one, two, or three valence electrons in to its outermost
shell will tend to lose the electrons and thus is considered a metal.
What, then, is particular about metallic bonding? In fact, it is the
idea that a metal is an aggregate of positively charged cores surrounded by a “sea” of electrons (see Figure 4.8). In the metallic
structure, the electrons actually hold the ions in place; otherwise
they would attract each other. One can think of this cohesion as a
mass of solid balls bonded by a very strong liquid glue. As a result of
this metallic structure, the path of an electron in the metallic bond
is completely random around the aggregate of ions (see Figure 4.9).
Therefore, metals are of course good conductors because electrons
can easily move in any direction. Second, metals usually have a high
Figure 4.8
A metallic bond forms when atoms give up their
valence electrons, which then form an electron
sea.
Core
Figure 4.9
The path of an electron in a metallic structure.
+
+
+
+
+
+
+
+
+
Electron