Review of Atomic Bonding

Lecture 2
CE 5010 Modern Construction Materials
Prof. Ravindra Gettu
IIT Madras
Elements
Mendeleyev (or Mendeléef) Periodic Table
Elements

Mendeleyev Periodic Table

• A way of presenting all the elements so as to show their

similarities and differences.
• The elements are arranged in increasing order of the
atomic number (from left to right).
• The horizontal rows are called periods and the vertical
columns are called groups.
• The elements of a group have a similar electronic
configuration. The number of electrons in the outer shell
is the same as the group number.
 Elements Evolution of
atomic models

The Bohr model, though still of

considerable use, has been modified.
Instead of definite orbits, the term orbital
is used to represent the distribution of
the electron within the space occupied
by the atom.
In the modern idea of the atom, the
electron, instead of having a fixed orbit,
is indicated by a probability of it
occupying any location. This is
illustrated below for hydrogen.

Higgins
Elements
Electronic Configuration
(Simplified Bohr model)

H C O Na Al Si

Cl Ar Ca Fe
Elements
Electronic Configuration
• The electrons in the outermost shell are responsible for
bonding with other atoms, and are called valence
electrons.

• The atomic structure is stable when the outermost shell

contains eight electrons (octet configuration).

• The number of electrons in the outermost shell

determines the valency (i.e., number of bonds which can
be formed with other atoms). Valency is the number of
valence electrons to be lost or gained to reach the octet
configuration.

Illston & Domone

Elements
Electronic Configuration
Interaction of Atoms
Interatomic forces

• When atoms are far from each other, there is a weak attraction
between them. The attractive force (a) increases as the interatomic
distance decreases.
• At close range, a repulsive force (b) builds up and ultimately balances
the attractive force.

Higgins
Interatomic Bonds
Primary Bonding Forces
• Ionic
• Covalent
• Metallic

Secondary Bonding Forces

• Van der Waals
• Hydrogen bonding
Ionic Bonding: NaCl

•• ••
Na • + Cl → Na+ [ Cl ]
••

••
••
•

•• ••

Sodium has 1 valence electron (valency = +1) and chlorine has 7 valence
electrons (valency = -1). By the transfer of 1 electron fron Na to Cl, both
attain the octet configuration.
However, the electrical neutrality of the atoms is disturbed, and Na
becomes positively charged and Cl becomes negatively charged ions
(Na+ & Cl¯).
The overall neutrality of the material is maintained.
Ionic (or Electrovalent) Bonding
• The atom can attain the octet configuration by losing or
gaining electrons. An ionic bond is formed when all the
bonded atoms realise their octet configurations by
donating/borrowing valence electrons. (Can only occur
between atoms of different elements.)

• The atoms are attracted to each other by electrostatic

forces.

• The strength of the bond between atoms A and B is

proportional to eAeB/r, where eA and eB are the charges
on the atoms, and r is the interatomic separation.

Illston & Domone

 Ionic Bonding
• Ionic bonds are strong; the strength increases when
two or more electrons are transferred.

• The strength of the ionic bond is reflected in properties

such as the melting point.

• The melting point of NaCl is 801°C (1 electron transfer),

that of MgO is 2640°C (2 electron transfer) and that of
ZrC is 3500°C (4 electron transfer).

• The ionic bond is non-directional;

i.e., the ions are arranged symmetrically.

Face-centred-
cubic structure
Illston & Domone
Covalent Bonding
Where atoms are of the electron acceptor type (i.e., -ve valency
or with close to 8 valence electrons), octet structures can be
attained by the sharing of 2 or more valence electrons between
the atoms.

This the basic mechanism of aggregation among elements of

Groups IV, V and VI.

The covalent bonding obeys the (8-N) rule,

where N is the number of valence electrons.
The maximum number of nearest neighbours
to each atom is 8-N.
Covalent Bonding

Covalent bonds are directional

O-Si-O angle: 109°

C-O & O-C-O angles: 180°

SiO2
 Covalent Bonding
Thus, chlorine (N=7) has only one neighbour; sulphur (N=6)
occurs in long chains; bismuth (N=5) occurs in long sheets; and
with carbon (N=4) a three-dimensional network can occur, as in
diamond.

One layer of graphite

 Covalent Bonding
• The covalent bond is saturated by the individual atoms
participating in it.

• Materials with covalent bonds do not have a three-

dimensional structure, with the exception of diamond
& silica.

• In chlorine, there is no extension of the covalent

bonding beyond the molecule. Similarly, in the chains
of sulphur and sheets of bismuth, there is no bond
between chains and sheets.

• Generally, covalent elements have poor strength, even

though the covalent bond is strong (as seen in
diamond). The chains coil in spirals leading to high
elasticity in some cases (e.g., rubber). Illston & Domone
Covalent Bonding
Carbon nanotubes

Nanotubes have carbon atoms in continuous

hexagon arrangements rolled into tube-like
structures that form tiny fibers 10 to 12 times
stronger than steel.

Nanotubes having strands narrower than a

human hair and 10 times stronger than steel
were first prepared in 1997.

Uses may include cables, sports equipment,

fabrics for bulletproof vests, and low-friction
bearings for micromachines.

nanotube
bundle
Metallic Bonding
• Metallic atoms have few valence electrons (i.e.,
elements of Groups I, II and III), and cannot bond with
themselves covalently.

• In a metallic crystal, the valence electrons are

detached or delocalised from their atoms and move
freely between the positive ions.

• The positive ions are arranged regularly in a crystal

lattice, and the electrostatic attraction between the
positive ions and the free negative electrons provides
the cohesive strength of the metal.

Illston & Domone

Metallic Bonding

The metallic bond can be considered as a special case

of the covalent bond, in which the octet structure is
attained by a generalised donation of the valence
electrons, which form a cloud that permeates
throughout the lattice.

Illston & Domone

Metallic Bonding
Metal structures are densely packed with atoms

12-coordination: Each atom in a structure has 12 touching neighbours. Each

atom has 6 atoms touching it in a layer and touches 3 atoms each of the layer
above and below.

8-coordination: Each atom in a structure has 8 touching neighbours. The

atoms do not touch each other within a layer but each atom touches 4 atoms
each in the layer above and below.
Metallic Bonding
• Since the electrostatic attraction between ions and
electrons is non-directional, metallic crystals can grow
in three dimensions.

• Metallic bonding leads to high thermal and electrical

conductivity, malleability and ductility in metals.

• High reflectivity and opacity of metals have also been

attributed to the absorption of energy by the free
electrons and subsequent emission of light when they
fall back to their original energy levels.

• The ability of metals to form alloys is also explained by

the free electron theory.
Illston & Domone
Van der Waals Bonding
• Weak bonds exist between atoms and molecules that
are called Van der Waals bonds.
Electrically symmetric atom

• The electron charge is spread around

the atom and over a period of time it is
symmetrically distributed. However, at
any instance in time, the electrostatic field Instantaneous distortion

is continuously fluctuating. This results in

the formation of dynamic electric dipoles
(i.e., the centres of the positive and
negative charges do not coincide).

Induced atomic dipole

Illston & Domone

Van der Waals Bonding
• When another atom is brought into proximity, the dipoles of the
two atoms interact resulting in a weak non-directional electrostatic
bond. The attractive force between two atoms
is:
F = α1 α2 / r 6
where α1 and α2 are the polarisabilities of
the two atoms (i.e., ease with which a
dipole can be formed), and r is the
distance between them.
Polarisability increases with atomic
number, since the valence electrons are
farther from the nucleus (can be more
easily pulled towards neighbouring
atoms).
• Van der Waals bonding is responsible for the viscosity and surface
tension of liquids.
• Heat can be used to break Van der Waals bonds; e.g., boiling of liquids,
melting of thermoplastic materials. Illston & Domone
Hydrogen Bonding
• The strongest of dipole interactions occurs
when the hydrogen atom is involved. This is
called the hydrogen bond.
Water bond Ice bond – open
crystal structure

• This is responsible for the high melting point of ice and

boiling point of water.
• Hydrogen bonding contributes to high mechanical
performance and heat resistance of some modern
polymers (e.g., nylon, Kevlar). Illston & Domone
Atomic Bonding: Summary of bond types

Young et al.
Mixed Bonding
In most materials, bonding is a mixture of two
(or even three) types:

– Plastics: covalent and Van der Waals

– Metallic alloys: covalent and metallic, Van der

Waals across grain boundaries

– Ceramics that may contain metallic and non-

metallic elements: may be covalently and
ionically bonded with Van der Waals bonds
across grain boundaries
Bonding Energies
Net attractive energy (Ucryst) versus distance (r) between the bonded ions
– Condon-Morse diagram

Strongly-bonded solids Weakly bonded solids

(ionic bond) (Van der Waals bond)
Young et al.
Bonding Energies

• Depth of the potential energy “well” (Umin) indicates the

strength of the cohesive forces within the solid

• The value of r that corresponds to Umin is the interatomic

distance r0 (i.e., equilibrium distance between ions in the
crystal)

• Primary bond lengths are in the range of 0.1-0.2 nm,

secondary bond lengths are in the range of 0.2-0.5 nm

• The boiling point of a material is proportional to the value of

Umin

Young et al.
Bonding Energies F

The bonding force (i.e., force of

attraction) between adjacent atoms
is
dU
F=
dr

The slope of the resulting curve U

about r0 is approximately linear and
gives a measure of the restoring
force that acts on the atoms for
small displacements from the
equilibrium position (Young’s
modulus of elasticity) Umin

Young et al.
Bonding Energies
Other consequences:

• Since the F(r) diagram is symmetrical at r0, the elastic

modulus is nearly the same in compression and
tension

• At large strains, the F(r) diagram is no longer straight;

response becomes nonlinear

• At high tensile strains, the material ruptures since the

attractive force reaches a maximum value (tensile
strength).

• There is no possibility of failure under pure

compression since the repulsive force always
increases. Illston and Domone
Bonding Energies
Effect of temperature on interatomic distances
Weak bond Strong bond

• The mean interatomic distance increases with temperature; only at

0 K (-273°C), Usolid = Umin.

• For a deeper well, the change in interatomic distance from r0 to r0´

is smaller, when temperature increases.

• Strongly bonded solids have lower thermal expansion. Young et al.

Bonding Energies
Other consequences:

• Higher interatomic separation leads to more vibration

of the atoms; as temperature increases, the material
expands in all directions

• If the heating continues, the atomic bonds are

eventually broken; liquids evaporate

• At higher temperatures, less extra energy is required to

break the bond; the tensile strength decreases with an
increase in temperature

Illston and Domone

 References
• The Science and Technology of Civil Engineering
Materials, J.F. Young, S. Mindess, R.J. Gray & A. Bentur,
Prentice Hall, 1998
• Construction Materials: Their nature and behaviour, Eds.
J.M. Illston and P.L.J. Domone, Spon Press, 2001
• Properties of Engineering Materials, R.A. Higgins,
Industrial Press, 1994
• http://www.webelements.com/