Civil Engineering Reference
In-Depth Information
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Metal ions
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Free electrons
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FIGURE 2.6
Metallic bonding. (©
Pearson Education, Inc. Used by
permission.)
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charged. An ion with a positive charge is a
cation
and one with a negative
charge is an
anion.
The ionic bond results from the electrostatic attraction of
the negatively and positively charged atoms. Since these bonds are based on
the transfer of electrons, they have no directional nature.
Covalent bonds occur when two similar atoms
share
electrons in the
outer subshell. The atomic orbitals of the atoms overlap and an electron in
each atom can exchange with an electron in its partner atom. If the shared
electron is considered to be attached to either of the atoms, that atom
would have the s and p subshells filled and would therefore be a stable
atom. Since the orbital paths of the atoms must overlap for the covalent
bond to form, these bonds are highly directional. In materials such as dia-
mond, the covalent bonds are very strong. However, the carbon chain
structure of polymers is also formed by covalent bonds and these elements
display a wide range of bond energy. The number of bonds that form
depends on the number of valence electrons. An atom with
N
electrons in
the valence shell can bond with only 8
N
neighbors by sharing electrons
with them. When the number of electrons
N
equals 7, the atoms join in
pairs. When
N
equals 6, as in sulfur, long chains can form since the atom
can bond with two neighbors. When
N
equals 5, a layered structure can be
developed. If there are 4 valence electrons, three-dimensional covalent
bonds can result (e.g., the structure of carbon in diamonds) (Jastrzebski
1987). The calcium-silicate chain in portland cement concrete is based on
covalent bonds.
Most interatomic bonds are partially ionic and partially covalent, and
few compounds have pure covalent or ionic bonds. In general, the degree
of either type of bonding depends on the relative positions of the elements
in the periodic table. The wider the separation, the more ionic bonds form.
Since the electrons in ionic and covalent bonds are fixed to specific atoms,
these materials have good thermal and electrical insulation properties.
Metallic bonds are the result of the metallic atoms having loosely held
electrons in the outer s subshell. When similar metallic atoms interact,
the outer electrons are released and are free to float between the atoms.
Thus, the atoms are ions that are electrically balanced by the free elec-
trons. In other words, the free electrons disassociate themselves from the
original atom and do not get attached to another atom. Metallic bonds are
not directional, and the spacing of the ions depends on the physical char-
acteristics of the atoms. The atoms tend to pack together to give simple, high
density structures, like marbles shaken down in a box. The easy movement
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