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June 12, 2009
From Atoms and Molecules to Nanoscale Materials
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Table 4.2 Types of molecular interactions.
Weak Interactions
Strong Interactions
(Secondary bonding)
(Primary bonding)
−1
−1
20–200 kcal mol
0.1–5 kcal mol
Covalent bonding
Electrostatic interaction
van der Waals forces
Ionic bonding
Metallic bonding
Dipole-dipole interaction
London dispersion forces
Hydrogen bonding
fundamental unit of all matter, the properties of matter are con-
trolled principally by the interactions between these units.
4.2 MOLECULES AND MOLECULAR INTERACTIONS
There are many types of interactions that hold atoms together,
and they may be broadly classified into primary or secondary
bonding on the basis of their strengths (Table 4.2). Covalent bond-
ing involves the sharing of electrons between two atoms. Air
consists mainly of N 2 molecules that are formed by two nitrogen
atoms covalently bonded together. Molecules are essential entities
making up most of our surroundings — e.g. air and water. The
forces of attraction that hold a molecule together are referred to as
intramolecular interactions.
4.2.1 Molecular Orbital Theory ch04
In a simplified manner, intramolecular bonding can be seen as the
overlap of AOs of two atoms, resulting in a higher electron density
in regions shared by the two nuclei (Fig. 4.2(a)). For example,
two p orbitals may overlap either head-on or sideways to produce
the σ or π bond respectively (Figs. 4.2(b) and (c)). In order for
the orbital overlap to yield effective interaction, the orbitals must
approach each other in the right orientation (Fig. 4.2(d)). The
strength of the resultant bond depends on the extent of overlap,
which in turn is affected by the symmetry and relative energy of
the two interacting orbitals.