Introduction to the Structure and Properties of Matter by gregorio11


									           Introduction to the Structure and Properties of Matter
For our purposes the fundamental unit of matter at the microscopic level is the atom,
ion or molecule. Recall that an ion is an atom that has gained or lost electron(s), while
a molecule is a strongly bonded (small) group of atoms which we regard as
indivisible. Matter, as we know it, at the macroscopic level consists of very large
numbers of these fundamental particles “held together” in some way so that we
recognise the substance as existing in one of the three phases of matter.

The phases of matter may be categorised by the well-known characteristics:

Gas            Compressible           Flows (fluid)

Liquid         Incompressible         Flows (fluid)

Solid          Incompressible         Does not flow

Testing these characteristics involves the use of physical measurement:

For compressibility we apply a Pressure and measure any Volume change

For flow we apply a Stress (force) and measure any velocity gradient (viscosity).

As we shall see these characteristics are not wholly true e.g. solids are compressible,
though to a far lesser extent than gases.

We also know that, for most materials, if we change the temperature we will change
the phase of the matter; so we induce freezing and melting or condensation and
boiling. It is also true that, under standard conditions, gases have very low densities,
liquids have much higher densities and solid densities are generally similar or larger
than those of liquids.

Thus we observe the close links between these physical quantities and the structure/
properties of matter. This is what Thermodynamics is all about! Note that all of these
measurements are made at the macroscopic level and we need know nothing about
what the matter is like at the microscopic level in order to derive equations linking
these parameters together and thus to explain and predict the behaviour of the matter.

However, to get a full and proper picture it is essential to link the microscopic and
macroscopic worlds together.

The crucial factor at the atomic level must be the existence of forces between the
particles and the nature of these forces. The second factor we should consider is the
energies of the particles, themselves. We are, in fact, trying to apply principles of
mechanics to these particles but will not succeed, as there are so many of them! We
find we must invoke the ideas of statistics to properly analyse matter at the
microscopic level; this is the subject of Statistical Mechanics.

Although we will not go as far as that in this discussion, it is helpful to look at matter
in terms of force and energy, in a descriptive manner.
Inter-particle forces and energy
We can summarise the strength of forces between particle in the different phases of
matter and the way energy is held in these phases, through the following table:

          Phase                         Forces                         Energy

                              No inter-particle forces but All the energy is held as
           Gas                phase must be constrained translational kinetic
                              in some volume by energy by the particles
                              external forces (pressure)
                              which will determine the

                              Inter-particle forces are      Potential energy from the
          Liquid              strong enough to hold          weak bonds between the
                              material together in its       particles with some kinetic
                              own volume with defined        energy (vibration and
                              density.                       translation)    -    which
                                                             facilitates bond breaking
                                                             and fluidity.

                              Strong inter-particle forces   Most of the energy is held
           Solid              hold    matter     together,   as potential energy due to
                              usually in a rigid 3-D         strong bonds. Particle
                              arrangement:     crystalline   motion exists as vibrations
                              solid of definite volume       within the crystal which
                              and density.                   hold kinetic energy.

This shows us that the phase of matter is linked to the relative amounts of kinetic and
potential energy in the system. If we raise the temperature by putting heat energy into
the matter (or lower the temperature by cooling it) we are changing the total internal
energy of the system. How is this extra energy held in the system? It can only exist as
kinetic energy since the potential energy component is linked wholly to the inter-
particle bonds. Hence, changes in the phase of matter result from changes to the way
the internal energy is held.

We need to discuss two aspects of the microscopic picture in more detail:

   •   The nature of the inter-particle bond; this is best done in the context of solids

   •   The link between the kinetic energy of the particles and the basic
       thermodynamics of the system; this is covered by the kinetic theory of gases.

This seems to leave liquids out in the cold! We must do this to some extent as they are
quite hard systems to understand in detail, but hopefully we can discuss some of their
properties such as viscosity and surface tension, later.

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