Atomic bonding &

crystalline structure

Dr. Sherif Refat Abd Elmonem

Lecturer of Dental
Biomaterials
 Revision:
• 1-Silicon impression material is a bioactive material:
• -TRUE
• -FALSE

False
2- which of the following is considered a good biocompatibility
response:
• - No foreign body reaction
• - pathological healing
• - cytotoxicity
• - no bacterial colonization

No bacterial colonization
- At large distances, interactions are
negligible because the atoms are too far
apart

- at small separation distances, each atom exerts

forces on other atoms.

-These forces are of two types:

attractive (FA)
and
repulsive (FR),
the magnitude of each force depends on the
separation or interatomic distance (r)
• The origin of an attractive force FA
depends on the type of bonding
between the two atoms.
• Repulsive forces arise from
interactions between the
negatively charged electron clouds
of the two atoms FR .

• The net force FN between the two

atoms is just the sum of both
attractive and repulsive
components; that is
FN = FA + FR
 • Q: when is the result of net force FN =
zero???

- When FA and FR are equal in magnitude but

opposite in sign

- Then the centers of the two atoms are

separated by the equilibrium spacing r , 0

as indicated in the Figure

 Relation between
interatomic bonding and
material properties
• The atoms are in a constant state of vibration, and
the average amplitude of vibration is dependent on
the temperature
• the higher the temperature the greater is the
amplitude the greater is the kinetic energy of
vibration

At a certain temperature, the minimal energy required

to maintain equilibrium distance between atoms is
detected at the bottom of the trough
the linear coefficient of
thermal expansion (α) of
materials tends to be
inversely proportional to
the melting temperature.
Q: Which of the two materials
do you expect to have higher
melting temperature
and
which has higher coefficient of
thermal expansion???
The tangent of the resultant force at equilibrium is
the slope of the curve which is the modulus of
elasticity (stiffness)
• MCQ:
• So, a high melting point
is usually accompanied
by a (greater/lower)
stiffness
 Crystal Structures

• A crystalline material is one in which the

atoms are situated in a repeating three-
dimensional pattern.
• All metals, many ceramic materials, and
certain polymers form crystalline structures
• For materials that do not crystallize, this
long-range atomic order is absent; these are
called non-crystalline or amorphous
materials
 Complex crystal structures are
displayed by some of the ceramic and
polymeric materials
The present discussion deals with
several common crystal structures for
metallic elements or alloys.
Crystalline metallic solids
• When crystalline structures are
described, atoms (or ions) are
described as solid spheres having
well-defined diameters.
• This is termed the atomic hard-
sphere model in which spheres
representing nearest-neighbor
atoms touch one another.
• An example of the hard-sphere
model is displayed in Figure
• Sometimes the term lattice is used to
describe crystal structures; lattice
means a three-dimensional
arrangement of atom positions(or
sphere centers).

• UNIT CELLS

• The atomic order in crystalline solids

indicates that small groups of atoms
form a repetitive pattern.

• unit cells is the building block of the

crystal structure
CRYSTAL SYSTEMS:
1.Because there are many different possible crystal structures, it is
sometimes convenient to divide them into groups according to unit cell
configurations and/or atomic arrangements.

2.One such scheme is based on the unit cell geometry, that is, the shape
of the appropriate unit cell parallelepiped without regard to the atomic
positions in the cell.
- xyz coordinate system is described
with its origin at one of the unit cell
corners

• each of the x, y, and z axes

coincides with one of the three unit
cell edges that extend from this
corner, as illustrated in Figure.
• The unit cell geometry is
completely defined in terms of six
parameters: the three edge lengths
a, b, and c,
and
• the three interaxial angles .
These are indicated
1.On this basis there are seven different possible combinations of a, b,
and c and, each of which represents a definite crystal
system.
2.These seven crystal systems are cubic, tetragonal, hexagonal,
orthorhombic, rhombohedral, monoclinic, and triclinic.
For reading only
1.Also, for metals, when we use the hard-sphere model for the crystal
structure, each sphere represents an ion core.
2.Three relatively simple crystal structures are found for most of the
common metals:
A- face-centered cubic
B- Body-centered cubic
C-hexagonal close-packed.
Cubic system
and its
modifications:
Cubic system:
• The cubic space lattice is characterized
by having axes that have equal lengths
and they meet at right angle
• This structure is theoretical for pure
metals
• each atom at each of the eight corners of
the cube is associated with eight
surrounding unit cells.
• Therefore, each atom is participating in
eight unit cells.
• each atom has 1/8 of its volume in each
of these eight cells

• The simple cubic structure contains one

metal atom per unit cell
• The Face-Centered Cubic Crystal
Structure:
• The crystal structure for many
metals has a unit cell of cubic
geometry,
• with atoms located at each of
the corners and the centers of
all the cube faces. It is called
the face-centered cubic (FCC)
crystal structure
• Some of metals having this
crystal structure are copper,
aluminum, silver, and gold
• the cube edge length a and the atomic
radius R are related through in FCC
• For the FCC crystal structure, there are eight corner
atoms (Nc 8), six face atoms (Nf 6), and no interior atoms
(Ni 0). So 4 atoms are present in FCC unit cell,
• The Body-Centered Cubic Crystal
Structure:
• Another common metallic crystal
structure also has a cubic unit cell
with:
atoms located at all eight corners and a
single atom at the cube center

• This is called a body-centered cubic

(BCC) crystal structure.
 Hexagonal
crystal system
and its
modification
Hexagonal crystals:
Simple hexagonal system:
• In which a = b ≠ C with γ = 120°
and α = β = 90°.
• The atoms at the corners with one
atom at the upper face and another
at the lower face
• At each corner of the hexagonal
system the atom value can be
considered:
• 1/6 and at the face as 1/2 atom
• so the value of simple hexagonal
unit cell is three atoms.
 Yet, metals
do not crystallize in the simple hexagonal pattern
because the packing factor of this system is too low
which represents less stable condition

Instead a more closed packed structure is formed

Closed packed hexagonal (HCP):
It can be considered as simple hexagonal system with
three unshared atoms at the same plane in the center
of the hexagonal system.
,

Link for animation https://images.app.goo.gl/oncwqp3f1FhZQUtL6

 Some of the space of the structural
unit is not occupied by the atoms.
The fraction
of space occupied by the atoms is
Definition of atomic packing factor called the atomic packing factor
(APF)and is calculated by:

For the FCC structure, the atomic

packing factor is 0.74, which is the
maximum packing possible for spheres
all having the same diameter.

Metals typically have relatively large the atomic packing factor for the HCP crystal structure is the
atomic packing factors same as for FCC = 0.74

Atomic packing factor for BCC is about : 0.68

 Imperfections in crystalline solids
The theoretical strength of crystals is always much higher than the
actual strength found from experimental work.

This is because nature is not perfect

materials are bound to contain some defects or imperfections that decrease
the strength.
Types of crystalline imperfections:
A-Point defects:
The simplest point defects are:

(i) Vacancy in which an atom is missing within a crystal.

CAUSED BY imperfect packing or thermal vibrations of the atoms at
elevated temperatures.
Vacancies may be single, two or more.

(ii) Interstitial impurities in which an extra atom may be lodged within a

crystal structure.

Such imperfections produce atomic distortion within the crystal lattice.

B-Line defects:

• The most common type of line

defect within a crystal is a
dislocation.
• Dislocation is defined as the
displacement of a raw of atoms
from their normal positions in the
lattice.
C-Plane defects:

• Such as grain boundaries in

metals
NONCRYSTALLINE (Amorphous) SOLIDS
There are many solids in which the
molecules are distributed at random
without regularity.

These solids are termed Amorphous,

which means without shape.

They do not have a definite melting

temperature, but rather they gradually Crystalline vs. Amorphous solids
soften as the temperature is raised and
gradually harden as they cool.

The temperature at which they first form

a rigid mass upon cooling or soften upon Below Tg, the material loses its fluid characteristics. Many
heating is called glass transition synthetic dental resins often have glassy structures, with Tg
temperature Tg. greater than body temperature
References