Poetry of Physics and the Physics of Poetry

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Chapter 10

Thermodynamics and the Atomic

and Molecular Structure of Matter

Both chemists and physicists have contributed to our understanding of
the two interrelated subjects we will study in this chapter, namely
thermodynamics and the atomic and molecular structure of matter. The
word thermodynamics in ancient Greek means literally the movement of
heat. This subject is intimately related to an understanding of the
gaseous, liquid and solid phases of matter and the transitions between
them in terms of the atoms and molecules of which they are composed.
Let us therefore begin our discussion by considering a solid, which as
we read earlier in Chapter 8 is not quite solid since the atoms of which it
is composed are mostly empty space. The spacing between atoms or
molecules if we are dealing with a compound is comparable with the size
of the atoms or the molecules themselves. The reason that a solid seems
solid is that there exists a binding force between each molecule in the
solid. The force arises from the uneven distribution of charge within each
molecule or atom. The positively charged nucleus of one atom attracts
the electrons of the other atom and vice-versa. But there are repulsive
forces as well between all the positively charged nuclei and also between
all the negatively charged electrons. On the average, the forces due to
these charges cancel each other but a slight residue remains, which
accounts for the molecular forces felt between the individual molecules
and atoms of matter. The sign of the force depends on the distance
between the molecules. There is an equilibrium position where all the
electric forces cancel but if the molecules are separated by a distance
greater than the equilibrium distance then the force between them is
attractive. If on the other hand, the molecules are closer than the
equilibrium position then the force is repulsive. It is the repulsive aspect
of the molecular force, which prevents us from compressing solids into
extremely small spaces. If you wish to experience this force then close