One of the first theories to describe the world around us said that everything is made of earth, air, fire, and water. If something had extra earth it would fall. If an object had extra air it would rise. This system could explain observations and was a good model for a long time. Even today there are some more sophisticated versions of this that are put forward. They don't think that there are such things as "atoms", but everything that is observed can be expained using this system. Maybe they are correct!
The Greek philosopher Democritus seems to be the first to suggest that matter is made up of small pieces or "atoms". He actually thought that everything, even the human soul, was made up of these small atoms. But he didn't have any idea about what the atom was; there wasn't a model for an atom, just an idea that they existed.
A very good scientist, John Dalton, made observations suggesting that gases are made up of small individual units. He was adding different gases that reacted with each other together and observing how much of the new substance was formed. He was observing chemical changes. His observation was that the gases always reacted in whole number ratios. As an example take the reaction of hydrogen and oxygen to form water. We know the formula for water is H2O and that each little piece of water has two hydrogens and one oxygen. If Dalton was mixing hydrogen and oxygen he would have found out that he needed twice as much hydrogen as he did oxygen in order to convert all of the hydrogen and oxygen to water. In other words he would have found that hydrogen and oxygen react in the whole number ratio of two to one. All of his work suggested that gases react in small whole number ratios. In order for this to happen the gases must be made of small individual units that also reacted in small whole number ratios. The makeup of these small units was not known, but it was becoming very clear that some kind of "atom" existed (Google Search). This work was done around the year 1803.
As we got into the 1800's the idea of an atom became more popular. The evidence was beginning to support the idea. A man named Robert Brown observed that particles suspended in water seemed to be bouncing about. From his observations of large objects and from knowing Newton's laws he suggested that they could only be moving if something was hitting them. This led him to believe that water was made of small pieces that could hit against the suspended particles. He didn't have any idea what these small pieces looked like, but it did suggest that matter is made of a collection of individual units. This motion of suspended particles in liquids is called Brownian motion (Google Search). This occured around the year 1827.
This section will help you:
- Understand some of the history of atomic theories.
- Know what the raisin pudding model is.
- Predict experimental results using this atomic theory.
Just before 1900 the existance of the electron was becoming known and during the early 1900's the electron was characterized by a very talented scientist by the name of Robert Millikan who performed the famous Millikan oil-drop experiment (Google Search). He and others found out that the electron was extremely small and that it has a negative charge.
With the discovery of the electron a model of the atom could be constructed. Since they knew that the electrons were negatively charged and that most materials are neutral, they had to postulate something with a positive charge that would balance the charge of the electron. What they proposed was a fluid-like substance with a positive charge that the electrons could be scattered around in. The electons would be like the raisins in a raisin pudding. The pudding would be the positve part of the atom and the electrons would be suspended around the atom in this positive "pudding" material. This atomic theory first seems to be proposed by J. J. Thompson around 1898 and is called the raisin pudding model (Google Search). Notice that there isn't any nucleus or other "large" object in this model of the atom, the only particles are the very small electrons.