The Structure of Matter
The Greeks were the first to speculate that matter was discrete, in the form of particles. The word atom derives from the Greek (ατομοζ) for indivisible. Democretus, argued that matter on the large scale is composed of atoms and that different substances were composed of different atoms or combination's of atoms. Furthermore, one could substance could be converted into another simply be re-arranging the atoms. The atomic theory was roundly rejected by Aristotle, and, thus, by almost everybody else for the next two millennia.
The modern definition of an element was made in 1661 by Robert Boyle. An element is a substance that can not be broken down into simpler substances but can form compounds with other elements. There are 88 naturally occurring elements (not the much reported 92 natural elements - The elements Tc, Pm, At and Fr have no stable isotopes, and none of long half-life, so they are not naturally present.) Including man-made elements, at the time of writting (Dec, 2006) there are 117 elements. The existence of these more massive elements is fleeting with elements lasting from a few microseconds to about 30 seconds.
For the Greeks, atoms were as far as the indivisibility of matter went. However, in 1906 J. J. Thompson discovered a negatively charged particle which eventually became known as an electron. Early models the atom considered 'atom as a nice hard fellows, red or gray in color, according to taste', in which the charged particles were distributed much like the plums in a Plum pudding. However, this model of the atoms was shown to be wrong by Rutherford's experiment, in which a high energy beam of alpha particles was fired at a very thin gold foil.
If the plum pudding model of the atom was correct then the alpha particles would pass through the foil with little deflection. As shown in the figure.
Most of the alpha particle passed through the foil with very little deflection. However, about 1 in every 8000 was scattered through an angle of more than 90 degrees. To Rutherford this was incredible. “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue-paper and it came back and hit you.” For the alpha particles to be scattered through such large angles and even coming back on themselves, they had to encounter a massive concentration of charged particles of very small size. The back scattering of alpha particles showed that most of the mass of the atom was concentrated at the nucleus.