A. Atoms and Electrons
If we had a superpowerful microscope and could zoom into the atom and molecule to see how they are arranged and how they hold themselves together what would we see? What does an atom or molecule really look like? An Atom is the smallest particle of an element that can exist alone or in combination. It is super-small; so small in fact, that we cannot see it even with the most powerful microscope in the world. Years ago scientist used to imagine that atoms were shaped pretty much like our solar system is shaped; with its nucleus like the sun and its electrons like the planetary system. There are big differences not only in size but also in charges. Whereas the proton and electron attract each other, like the sun and planets, the electrons repel each other. The planets pull each other. The atom’s oppositely charged particles, electron and proton, pull each other and keep the atom’s overall charge at zero. This is the electrostatic force which is much greater then the gravitational forces. An atom has as many electrons around it as it has protons. Here the charge is zero. This is not the case with an ion. An ion is an atom that has either lost an electron or captured an extra electron. Thus an ion always has a charge, positive or negative.
Electrons arrange themselves around the atoms in orbits or shells. Each shell can receive only a limited number of electrons. For example the inner shell can hold no more than two electrons. The next shell can hold no more than eight electrons. Once a particular shell is filled then any new electrons must start on the next level or shell.
In this diagram the inner dark circle represents the nucleus and the outer circles are the shells with the electrons.The number is the limit of electrons in that shell.
(figure available in print form)
In this diagram the inner dark circle represents the nucleus and the outer circles are the shells with the electrons. The number is the limit of electrons in that shell.
In the above representation the atom has three complete electron shells for a total of 18 electrons. The number of electrons that an atom has is also its identifying atomic number. In this case the atom (element) with atomic number 18 is Argon. The Periodic Table of Elements is arranged numerically. Hydrogen is assigned atomic number one because it has only on electron. Helium is number two because because it has two electrons and so forth.
Some atoms like to have stability. This means that they like having their shells filled. They tend to do this by either gaining or losing electron with other atoms in whose proximity they may happen to be in. Thus, for example, chlorine, lacking one electron to fill its outermost occupied shell, might tend to pick up that electron from, say, sodium in whose proximity it happens to be in. Sodium, with only one electron in its outermost occupied shell might easily lose that electron in order to leave that shell empty and expose the filled shell beneath as its outermost occupied shell. The diagram below demonstrates this more clearly:
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(figure available in print form)
Atoms combine in this way to gain stability and in the process create new compounds. In this case the ions of chlorine and sodium combine to form sodium chloride, common table salt.
B. Crystal Forms
In a free flowing solution the ions, atoms or molecules attract each other until they bond. They will continue bonding, linking up layer after layer until they have no more room to grow or the solution is no longer saturated. You may have many crystals in a solution growing at the same time. These crystals may eventually meet. Where they meet and join, called the grain boundary, they stop growing as individual crystals but may continue growing as a polycrystal.
In this diagram four crystals are growing against each other. The dotted lines are the layers of ions, atoms or molecules. The grain boundary is evident.
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(figure available in print form)
Nearly all rocks are made up of crystals. The different kinds of crystals can often be distinguished from each other. This is especially true if the rock is polished.
Metal objects are made up of interlocking crystals. Sometimes you can clearly see their boundaries. For example in the zinc coating of galvanizes garbage cans. Polished brass door knobs often show their grain boundaries.
Few substances are not crystalline in nature. One of these is glass. Window glass and volcanic glass are examples. There is a glassy candy that comes with nuts in it that is another example. In these cases the solutions are cooled too quickly to allow the molecules to move into their proper crystalline structure. The molecules become frozen, locked into a patternless structure. It is conceivable that after many years (millions?) the molecular pull will rearrange the molecules into their proper places and the glass will then revert to a crystalline form. This is the reason why very old glass is not found.
