Types of Atomic &
Molecular Bonds
Types of Atomic & Molecular
Bonds
Objectives of the class
Develop a sense of history of scientific thinking
Explain the types of bonds
You should be able to have develop simple models for
the mechanical properties based on the type of
bond
Long Tradition in Science
Greek philosopher Leucippus used the word
atom, which means indivisible (400 BC)
Lots of work done by the alchemists
Lavoisier: chemical reaction leads to no net
loss or gain of matter.
Dalton: concept of atomic mass (1808)
Mendelev: periodic table of elements (1869)
Moseley: showed that the number of electrons
= number of protons (atomic number)
Discovery of X-rays
Wilhelm Conrad Roentgen
first Nobel prize in physics (1901)
Long Tradition in Science
(cont.)
Ruterford: experiments with alpha
particle (1907-11)
Max Planck: quanta (1900)
Louis de Broglie: all matter has both
wave and particle properties
(1923).
Types of Atomic & Molecular
Bonds
Primary Atomic Bonds
IMPORTANT!!!
Ionic Bonds
Covalent Bonds
Metallic Bonds
Secondary Atomic & Molecular
Bonds
Permanent Dipole Bonds
Fluctuating Dipole Bonds
Ionic Bonding
Large interatomic forces are created by the
“coulombic” effect produced by positively
and negatively charged ions.
Ionic bonds are “nondirectional”.
The “cation” has a + charge & the “anion” has
the - charge.
The cation is much smaller than the
17
anion. 11
Historical Note
Greek philosopher Empedocles (500 BC)
suggested that chemical changes are
caused by an emotional likes and
dislikes.
The love between two substances will
make them unite and form a third
substance. On the other hand, if the
substances start to hate each other
they will decompose.
Force vs. Separation Distance
2
Fatra
Frep
Z1 Z 2 e
=−
2
4πε a
nb
= − n +1
a
Simple example
Calculate the coulombic force of
attraction between Na+ and
Cl- in NaCl.
Note
rNa + = 0.098nm
rCl− = 0.181nm
Energy vs. Separation Distance
example
Modulus of Elasticity
Effect of temperature
Ionic Bonding in Solids
Because the ionic bond is
nondirectional the ions pack
together in a solid in ways
which are governed by their
relative sizes.
Another important factor is that
the ions must be arranged so
that their is local charge
neutrality. [Note the structure
of NaCl.].
Covalent Bonding
Large interatomic forces are created by the sharing of
electrons to form directional bonds.
The atoms have small differences in electronegativity
& close to each other in the periodic table.
The atoms share their outer s and p electrons so that
each atom attains the noble-gas electron
configuration.
Representations
discrete
cloud
Covalent Bonding in Carbon
A carbon atom can form form sp3
orbitals directed symmetrically
toward the corners of a
tetrahedron. [Note the examples
below.]
Diamond
Metallic Bonding
Large interatomic forces are created by the
sharing of electrons in a delocalized
manner to form strong nondirectional
bonding.
Metalic Bond
It is convenient for many purposes to regard an atom
in a metal as having a definite size, which may be
defined by the distance between its center and that
of its neighbor.
This distance is that at which the various forces acting
on the atom are in equilibrium.
In a metal, the forces can be considered as
–  (a) the attractive forces between electrons &
positive ions,
–  (b) the repulsion between the complete electron
shells of the positive ions, &
–  (c) the repulsion between the positive ions as a
result of their similar positive charges.
The Hard Sphere Model
This approach can be called the "hard sphere" model
of an atom.
The radius of an atom (or ion) determined for a
particular crystal structure is not a real characteristic
of that atom, because when the same atom appears
in different crystal structure it displays different
radii.
The radius of an atom (or ion) can be determined for a
particular metal by using the dimensions of the unit
cell of the crystal structure it forms.
Secondary Atomic & Molecular Bonds
[Van der Waals Bonds]
Permanent Dipole Bonds
–  Weak intermolecular bonds are formed between molecules
which possess permanent dipoles.
–  A dipole exists in a molecule if there is asymmetry in its
electron density distribution.
Fluctuating Dipole Bonds
–  Weak electric dipole bonding can take place among atoms
due to an instantaneous asymmetrical distribution of
electron densities around their nuclei.
–  This type of bonding is termed fluctuation since the
electron density is continuously changing.
Polar dipole
Van der Walls bonds
Question
If ionic forces are so strong, how
come does salt (NaCl) dissolves so
quickly in water???
Water has a very interesting
structure
Why ice floats in liquid water?