HISTORICAL PERSPECTIVE

INTRODUCTION:
The structure of matter has long intrigued humans.  The seed of modern atomic theory started in the Greek times when it was thought that all matter was composed of four "elements" earth, air, water, and fire.  The term "atom" is derived from the Greek word atomos, meaning "indivisible."

John Dalton, an  English schoolmaster revived the concept of atoms and proposed an atomic theory based on facts and experimental evidence.  Dalton's atomic theory proposed in the early 1800's may be summed up as follows: 

1. Elements are composed of minute, indivisible particles called atoms.

2. Atoms of the same element are alike in mass and size.

3. Atoms of different elements have different masses and sizes.

4. Chemical compounds are formed by the union of two or more atoms of different elements.

5. When atoms combine to form compounds, they do so in simple numerical ratio.

6. Atoms of two elements may combine in different rations to form more than one compound.

Presently, some of his statements must be modified: (1) atoms are composed of subatomic particles; (2) all the atoms of a specific element do not have the same mass; and (3) atoms under special circumstances can be decomposed. 

Students should begin to understand that an atom is one unit, but you can also have subatomic particles of an atom.  They are called electrons, protons, and neutrons.  An electron is a particle with a negative electrical change; a proton is a particle with a  positive electrical charge; and a neutron is a neutral particles. 

There are many ways to represent these structures to students.  In the next three labs, students will do the following to try and give them a conceptual feeling for atomic theories.  You must emphasize with students that understanding the internal workings of an atom is difficult.  Experimental evidence guides the theories, but it is subject to change.  Your students are only getting the beginnings.

Look at the periodic table of elements and construct a geometric crystal shape of what some atoms look like.  Try and figure out if the crystalline structure is unique to a specific element, or is it the subatomic particles.