Lecture 3 : physics of materials
2nd class/poly.Dep.
Bonds in Materials:
We see in the previous lectures , the materials can classify to three state
:*solid , **liquid, ***and gas, then the materials consist of many of
atoms which between together by bonds with different shapes such as
crystalline system and non-crystalline system.
Now must define the types of bonds, and how can effect in properties of
each materials.?
Solids can be classified according to the nature of the bonding between
their atomic or molecular components. The traditional classification
distinguishes four kinds of bonding:[1]




Covalent bonding, which forms network covalent solids
(sometimes called simply "covalent solids")
Ionic bonding, which forms ionic solids
Metallic bonding, which forms metallic solids
Weak intermolecular bonding, which forms molecular solids
(sometimes anomalously called "covalent solids")
Typical members of these classes have distinctive electron distributions,
thermodynamic, electronic, and mechanical properties. In particular, the
binding energies of these interactions vary widely. Bonding in solids can
be of mixed or intermediate kinds, however, hence not all solids have
the typical properties of a particular class, and some can be described as
intermediate forms .
Basic classes of solids
A network covalent solid consists of atoms held together by a network of
covalent bonds (pairs of electrons shared between atoms of similar
electronegativity), and hence can be regarded as a single, large
molecule. The classic example is diamond; other examples include
silicon,[3] quartz and graphite.
Their strength, stiffness, and high melting points are consequences of
the strength and stiffness of the covalent bonds that hold them
together. They are also characteristically brittle because the directional
nature of covalent bonds strongly resists the shearing motions
associated with plastic flow, and are, in effect, broken when shear
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Lecture 3 : physics of materials
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occurs. This property results in brittleness for reasons studied in the field
of fracture mechanics. Network covalent solids vary from insulating to
semiconducting in their behavior, depending on the band gap of the
material.
A covalent bond is a chemical bond that involves the sharing of electron
pairs between atoms. The stable balance of attractive and repulsive
forces between atoms when they share electrons is known as covalent
bonding.[1] For many molecules, the sharing of electrons allows each
atom to attain the equivalent of a full outer shell, corresponding to a
stable electronic configuration. As in fig
Pairs of electrons located between atoms represent covalent bonds.
Multiple pairs represent multiple bonds, such as double bonds and
triple bonds. An alternative form of representation, not shown here, has
bond-forming electron pairs represented as solid lines.
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Lecture 3 : physics of materials
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Physical properties of covalent compounds :
Physical properties
States (at room temperature)
Electrical conductivity
Boiling point and Melting point
Solubility in water
Thermal conductivity
Covalent compounds
Solid, liquid, gas
Usually none
Varies, but usually lower than ionic compounds
Varies, but usually lower than ionic compounds
Usually low
Ionic solids
A standard ionic solid consists of atoms held together by ionic bonds,
that is, by the electrostatic attraction of opposite charges (the result of
transferring electrons from atoms with lower electronegativity to atoms
with higher electronegativity). Among the ionic solids are compounds
formed by alkali and alkaline earth metals in combination with halogens;
a classic example is table salt, sodium chloride.
Ionic solids are typically of intermediate strength and extremely brittle.
Melting points are typically moderately high, but some combinations of
molecular cations and anions yield an ionic liquid with a freezing point
below room temperature. Vapor pressures in all instances are
extraordinarily low; this is a consequence of the large energy required to
move a bare charge (or charge pair) from an ionic medium into free
space.
Ionic bonding is a type of chemical bond that involves the electrostatic
attraction between oppositely charged ions. These ions represent atoms
that have lost one or more electrons (known as cations) and atoms that
have gained one or more electrons (known as an anions). In the simplest
case, the cation is a metal atom and the anion is a nonmetal atom, but
these ions can be of a more complex nature, e.g. molecular ions like
NH4+ or SO42-
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Notice : Electronegativity, symbol χ, is a chemical property that
describes the tendency of an atom or a functional group to attract
electrons towards itself.
It is important to recognize that clean ionic bonding – in which one atom
"steals" an electron from another – cannot exist: All ionic compounds
have some degree of covalent bonding, or electron sharing. Thus, the
term "ionic bonding" is given when the ionic character is greater than
the covalent character—that is, a bond in which a large electronegativity
difference exists between the two atoms, causing the bonding to be
more polar (ionic) than in covalent bonding where electrons are shared
more equally. Bonds with partially ionic and partially covalent character
are called polar covalent bonds.
polar covalent bonds
Electrons are not always shared equally between two bonding atoms;
one atom might exert more of a force on the electron cloud than the
other. In which case can make a dipole-dipole intermolecular force.
Ionic compounds conduct electricity when molten or in solution, but
typically not as a solid. There are exceptions to this rule, such as
rubidium silver iodide, where the silver ion can be quite mobile. Ionic
compounds generally have a high melting point, depending on the
charge of the ions they consist of. The higher the charges the stronger
the cohesive forces and the higher the melting point. They also tend to
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Lecture 3 : physics of materials
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be soluble in water. Here, the opposite trend roughly holds: The weaker
the cohesive forces the greater the solubility.
Metallic solids
Metallic solids are held together by a high density of shared, delocalized
electrons, resulting in metallic bonding. Classic examples are metals such
as copper and aluminum, but some materials are metals in an electronic
sense but have negligible metallic bonding in a mechanical or
thermodynamic sense (see intermediate forms). Metallic solids have, by
definition, no band gap at the Fermi level and hence are conducting.
Solids with purely metallic bonding are characteristically ductile and, in
their pure forms, have low strength; melting points can be very low (e.g.,
Mercury melts at 234 K (−39°C). These properties are consequences of
the non-directional and non-polar nature of metallic bonding, which
allows atoms (and planes of atoms in a crystal lattice) to move past one
another without disrupting their bonding interactions. Metals can be
strengthened by introducing crystal defects (for example, by alloying)
that interfere with the motion of dislocations that mediate plastic
deformation. Further, some transition metals exhibit directional bonding
in addition to metallic bonding; this increases shear strength and
reduces ductility, imparting some of the characteristics of a covalent
solid (an intermediate case below).
metallic bonding
Metallic bonding occurs as a result of electromagnetism and describes
the electrostatic attractive force that occurs between conduction
electrons (in the form of an electron cloud of delocalized electrons) and
positively charged metal ions. It may be described as the sharing of free
electrons among a lattice of positively charged ions (cations). Metallic
bonding accounts for many physical properties of metals, such as
strength, ductility, thermal and electrical resistivity and conductivity,
opacity, and luster.
Metallic bonding is not the only type of chemical bonding a metal can
exhibit, even as a pure substance. For example, elemental gallium
consists of covalently-bound pairs of atoms in both liquid and solid
state—these pairs form a crystal lattice with metallic bonding between
them.
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Lecture 3 : physics of materials
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