therefore calledhard-core repulsion. If it is ions that approach each other,
the interaction is often called Born repulsion.
3.1 TYPES OF BONDS
The next five rows in Table 3.1 relate to what may be properly called bonds.
They vary widely in strength, also within one type. The strength is commonly
given in terms of energy, expressed in J per mole of bonds. (Note that the
expression of a strength in units of energy is quite unlike that for macroscopic
systems, where strength commonly refers to the force per unit cross-sectional
area needed to cause breaking, i.e., in N?m^2 .) In general, only covalent
bonds and some ion–ion bonds may be strong enough to give ‘‘permanent’’
single bonds. Permanent here means that the atoms or groups bonded stay in
this configuration for ordinary times (at least several seconds). If the bond
energy is small, thermal motion of the atoms tends to break the bonds within
a very short time. The average kinetic (i.e., thermal) energy of a molecule is of
the orderkBT; this is further discussed in Section 4.3.1. Therefore it is often
useful to give interaction energies relative tokBT, as is done in Table 3.1; at
room temperature 1 kJ?mol^1 & 0 : 4 kBT. If bond energy 5 kBT, bonds will
not be formed; if it is 4 kBT, permanent bonds will be formed.
A great number of weak bonds acting on the same molecules or
ensembles of molecules may also cause permanent bonding. Examples are
van der Waals bonds holding molecules in crystals and various weak bonds
keeping globular proteins in a compact conformation or keeping particles
flocculated.
Covalent Bonds. Covalent bonds exist if some electrons
participate in the orbitals of more than one atom. These bonds are highly
specific and are extensively discussed in texts on organic chemistry. Here it
may suffice to remark that covalent bonds may be very strongðmostly 150–
900 kJ?mol^1 Þand act over a very short distance in a very restricted range
of directions.
Coulomb Forces. These are also called electrostatic forces, ion–ion
bonds (if attractive), or charge–charge interactions. They always occur
between charged particles, be they ions, protein molecules, or colloidal
particles, and they are also quite strong. From Coulomb’s law we have for
two chargesz 1 andz 2 ,
F!z 1 z 2
er^2ð 3 : 1 Þwhereeis therelative dielectric constantof the medium. Since the force (F)is
proportional to the product of the charges, it is negative (attractive) if the