Atoms and Molecules
159
the Auger electron is not knocked out by the photo-electric absorption of a photon
emitted by the electron which undergoes a transition to the K shell, but emerges
directly as a part of the process of readjustment of the atom. For example, the
vacancy in the K shell may be filled by an electron in the L I shell and the electron
in the L II shell may be knocked out, with the result that there will be two vacancies
in the L shell. Thus, the de-excitation of the atom may be accompanied either by
the emission of a photon (characteristic radiation) or an electron (Auger electron).
The two processes together essentially account for the number of vacancies in
the K shell. Finally, it is noted that the basic process in the Auger effect is also
known as auto-ionization or internal conversion (in nuclear transitions).
5.6 MOLECULAR BONDING
When atoms approach one another, attractive forces come into play which
generally, though not always, bind them into molecules. The mechanism
generating these attractive forces differs from molecule to molecule, but is usually
a combination (i) van der Waals forces, (ii) ionic (or heteropolar) bonds, and
(iii) covalent (or homopolar) bonds. In most cases, however, the ionic bonds or
covalent bonds are dominant. Here, a brief discussion of these mechanisms is
given for diatomic melocules. It is to be noted that the molecular forces arise
primarily from the interaction of the outer electrons.
Van der Waals Forces
When two atoms approach each other, though neutral, they induce fluctuating
but correlated electric dipole moments in each other. This gives rise to a dipoledipole attractive potential which is of the form – a/r
6
. When the atoms are so
close that the electronic orbits overlap, the Pauli exclusion principle forces them
into higher orbits which essentially brings in strong repulsive forces. The effective
potential may be written in the form:
V (r) =
6
n
b
a
r
r
−
(5.58)
where a and b are positive numbers and n is a large number (~ 10). This represents
what is known as van der Waals interaction. Of course, when the nuclei are very
close to each other, the dominant force is the repulsion between the nuclei. However,
the forces in this region are not important for molecular bonding.
Ionic Bonds
These bonds are formed when it is energetically favourable for electrons to be
transferred from one atom to another, with the resulting ions held together by
electrostatic attraction between them. Such bonds are known as ionic or
heteropolar bonds and are exemplified by the bonds in NaCl, KBr, HCl, etc.
As a specific case, we consider the KCl molecule. The K atom has its
valence electron in the 4s shell with a binding energy of only 4.34 eV. Now, a Cl
159
the Auger electron is not knocked out by the photo-electric absorption of a photon
emitted by the electron which undergoes a transition to the K shell, but emerges
directly as a part of the process of readjustment of the atom. For example, the
vacancy in the K shell may be filled by an electron in the L I shell and the electron
in the L II shell may be knocked out, with the result that there will be two vacancies
in the L shell. Thus, the de-excitation of the atom may be accompanied either by
the emission of a photon (characteristic radiation) or an electron (Auger electron).
The two processes together essentially account for the number of vacancies in
the K shell. Finally, it is noted that the basic process in the Auger effect is also
known as auto-ionization or internal conversion (in nuclear transitions).
5.6 MOLECULAR BONDING
When atoms approach one another, attractive forces come into play which
generally, though not always, bind them into molecules. The mechanism
generating these attractive forces differs from molecule to molecule, but is usually
a combination (i) van der Waals forces, (ii) ionic (or heteropolar) bonds, and
(iii) covalent (or homopolar) bonds. In most cases, however, the ionic bonds or
covalent bonds are dominant. Here, a brief discussion of these mechanisms is
given for diatomic melocules. It is to be noted that the molecular forces arise
primarily from the interaction of the outer electrons.
Van der Waals Forces
When two atoms approach each other, though neutral, they induce fluctuating
but correlated electric dipole moments in each other. This gives rise to a dipoledipole attractive potential which is of the form – a/r
6
. When the atoms are so
close that the electronic orbits overlap, the Pauli exclusion principle forces them
into higher orbits which essentially brings in strong repulsive forces. The effective
potential may be written in the form:
V (r) =
6
n
b
a
r
r
−
(5.58)
where a and b are positive numbers and n is a large number (~ 10). This represents
what is known as van der Waals interaction. Of course, when the nuclei are very
close to each other, the dominant force is the repulsion between the nuclei. However,
the forces in this region are not important for molecular bonding.
Ionic Bonds
These bonds are formed when it is energetically favourable for electrons to be
transferred from one atom to another, with the resulting ions held together by
electrostatic attraction between them. Such bonds are known as ionic or
heteropolar bonds and are exemplified by the bonds in NaCl, KBr, HCl, etc.
As a specific case, we consider the KCl molecule. The K atom has its
valence electron in the 4s shell with a binding energy of only 4.34 eV. Now, a Cl