The Evolution of the Atomic Theory Rob CongroveB 4 10/23/00 The five atomic theory's of the past two centuries represent the sudden advancement of science in modern times. Begining with a basic theory on the behavior of atoms to the current model, some changes have been made, and some ideas are still the same. Ancient Greek philosophers be lived that everything was made up of invisible particles called at mos. Since then the theory of atoms did not progress untill 1803. John Dalton was the first scientist to compose a theory of matter based on atoms. Dalton's atomic theory is based on four concepts.

He stated: '1. All elements are composed of atoms, which are indivisible particles. 2. All atoms of the same element are exactly alike; in particular, they have the same mass. 3. Atoms of different elements are different; in particular, they have different masses.

4. Compounds are formed by the joining of atoms of two or more elements.' 1 All of Dalton's ideas account for the laws of definite and multiple proportions and the law of conservation of mass. Some of Dalton's points are still thought to be true, but over time this original theory has been modified. The first of these modifications came in 1897 when J. J. Thomson discovered the electron.

Based on the work of William Crookes and his 'Crookes tube' (Cathode-ray tube), Thomson discovered a negative charged particle was the cause of the light produced by the cathode-ray tube. He also discovered that these particles are present in all elements. These cathode-ray particles are now known as electrons. Soon after the discovery of electrons the proton was discovered. This led Thomson to conclude that ther were an equal number of both particles present in the atom. Twelve years later Lord Ernest Rutherford was experimenting with alpha particles.

He shot a stream of them at a price of gold foil surrounded by zinc-sulfide. When an alpha particle strikes Zus it produces a flash of light. The particles mostly stayed in a constant stream through the foil, but a few were deflected. This led Rutherford to be live that there must be a small, dense cluster of protons in the middle of the atoms to deflect the small number of particles. Neil's Bohr was the next physicist to advance the atomic theory.

He explained what Rutherford could not about how the electron could stay in orbit around the nucleus. When the electron has little energy it is closer to the nucleus, when it absorbs more energy it travels farther from the nucleus. There is a definite number of electrons that can be in the same orbit. Whenthe orbits closest to the nucleus are filled the atom is at a ground state. Whenthe electrons become charged they move into a higher orbit and are then at an exited state. When the electrons move into a closer orbit they release a photon of radiation.

This model for the atom became insufficient. The charged-cloud model was born. It was the research of many scientists that led to this. Instead of showing the position and orbit of the electron, it shows the most probable locations. The orbits from the Bohr model are divided into sub-orbits, but ther is no way of showing the infinite amount of sub-orbits or which orbit would be in so the most dense place in the cloud is the most probable location of an electron.

The Atomic theory has gone through many changes since Dalton's time, but two of his ideas are still true. What we know about atoms today could be proved entirely wrong just like Dalton. The atomic theory is still just a theory and some day it might be proved or disproved.