Download Atomic building blocks

By
Dr.Reham Mohammed Abdallah
Items to be covered
Types of Atomic Bonding
I. Primary bonds
 Ionic bond
 Covalent bond
 Metallic bond
II. Secondary bonds
 Hydrogen bond
 VanderWaal bond
Arrangement of atoms in solids
 Crystalline solids
 Non-crystalline solids

All materials are built up from atoms and
molecules and there is a strong relationship
between atomic basis of a material and its
properties.

To understand the properties of materials, we
need to have an understanding of the way
atoms can combine to make solids.
Types of Atomic Bonding
I. Primary bonds
1. Ionic bond

It is the attraction of positive and negative ions.

The classic example is sodium chloride (Na Cl )
because the sodium atom contains one valence
electron in its outer shell and the chlorine atom has
seven electrons in its outer shell.

The transfer of the sodium valence electron to the
chlorine atom results in the stable compound Na Cl.
(Fig.1)

Ionic materials are characteristically hard and
brittle.

Electrically and thermally insulators.

In dentistry ionic bond is found in gypsum
investment, phosphate cement and ceramic.
Fig.1: Ionic bond.
2. Covalent bond

Sharing of electrons between adjacent
atoms.
Example:
a
molecule
of
methane(CH4).

The carbon atom has four valence
electrons, whereas each of the four
hydrogen atoms has a single valence
electron (Fig.2).

Covalently bonded materials are typically electrical
and thermal insulators.

In dentistry: monomer molecules of dental resin are
held by covalent bonds within the polymer chain.
Fig.2: Covalent bond.
3. Metallic bond

“It is the attraction between +ve cores and free
electrons or electron cloud”.

It occurs in metals, because they easily give up the
electrons in their valence shells giving positives
cores.

The electrons move freely through the metal from
atom to atom and from electron cloud. There is
attraction between free electrons and the positively
charged cores.

The metallic bond is weaker than the ionic and the
covalent bonds (Fig.2.3).
This structure gives the metal:
1. Excellent thermal and electrical conductivity
2. Opacity(due to absorption of light by free electrons).
3. Lustrous appearance.
4.The ability to be plastically deformed.
Fig.2.3: Metallic bond.
II. Secondary bond

In contrast with primary bonds, secondary
bonds don’t share electrons. Instead, charge
variations among molecules or atomic groups
induce polar forces that attract the molecules .

These forces are physical, weak and arise from
the polarization of molecules i.e. formation of
electrical dipoles.
1. Permanent dipole

An electrical dipole or polarization exists in a
molecule that has an electrical imbalance and
hence has a center of positive charge and a
center of negative charge.

This type of attraction forces occurs in
asymmetric molecules where one atom can
attract the electrons and becomes negative, and
the other atom becomes positive in relation to it
resulting in a permanent dipoles.
Hydrogen bond

The hydrogen bond is an important example. In
H2O, there is a covalent bond because oxygen and
hydrogen atoms share electrons. However, the
electrons around oxygen nucleus are more than
those around the hydrogen nucleus and as a result
the hydrogen portion of the water molecule is
positive in relation to the oxygen portion.

Therefore “attraction will take place between the
positive hydrogen portion of one water molecule
and the negative oxygen portion of another water
molecule” (Fig.4).

Example: Absorption of water by synthetic
resin.
Fig.4: Hydrogen bridges formed between water
molecules.
2. Van der Waal bond (fluctuating dipole)

“it is the attraction between the +ve pole of
one atom and the –ve pole of another atom”. In
symmetric molecule, such as in inert gases, the
electron
field
is
constantly
fluctuating.
Normally, the electrons of the atoms are
distributed equally around the nucleus and
produce an electrostatic field around the atom.

However, this field may fluctuate and the
electrons may concentrate toward one side of
the atom and as a result its charge becomes
momentarily
positive
and
negative.
A fluctuating dipole thus created which will
attract other similar dipoles. Such interatomic
forces are quite weak (Fig.5).

Chemisorption of gases by alloy liquids is
followed by attraction due to vander Waal, forces.
Fig.5: Fluctuating dipoles.
A solid whose molecules are bonded together by
Van Der Waal forces has:
1- Low modulus of elasticity.
2- Low melting point.
3- High thermal expansion
e.g. waxes, acrylic resin
STRUCTURAL ARRANGEMENT OF ATOMS
IN SOLIDS

All solid materials may be classified on the
basis of their atom arrangements as:

Crystalline

Non crystalline or amorphous

Semi crystalline
1. Crystalline Solids
 Solid dental materials are termed crystalline
when their atoms are regularly arranged in a
space lattice. A space lattice is the regular
arrangement of atoms in the space so that
every atom is situated similary to every other
atom.
 (Fig.2.6).
Fig.6: Space lattice of
crystalline solid
material.

There are about 14 different types of space
lattice but only few are of dental interest. The
simplest way to study these types, is to
consider a unit cell which is the smallest
repeating unit in the space lattice.(fig.7)
II. Non-crystalline solids

Amorphous means without shape. Gases and
liquids are amorphous substances.

Some solids like glass and some polymers are
amorphous because of the random arrangement of
their atoms, yet their atoms may form a short
localized range of order lattice with a considerable
number of disordered units in between.

Since such an arrangement may be considered
typical of the liquid structure, these solids are
sometimes called “super cooled liquids”(Fig.9).
Crystalline solids
1) Have definite
Melting temperature.
2) Have regular unit
Cell with repetition.
Amorphous solids
1) No definite melting
Temp (gradually soften
on heating and gradually
harden on cooling)
2) No regular unit cell
But may have a short
range of regularity but
no repetition.