In 1808, a scientist named John Dalton published a paper which presented this set of four ideas:
- Each element is made up of tiny, tiny particles called atoms.
- Atoms of the same element are identical; atoms of different elements are different.
- When atoms combine together, they form chemical compounds.
- Chemical reactions change the way atoms are bound to each other. The atoms themselves are not changed in a chemical reaction, only the way they are connected to each other.
What does this mean? We can see that atoms must be tiny particles that are themselves unique. We cannot cut a piece of iron in half and form anything but two pieces of iron. By this we mean that because iron is composed of all iron atoms, we cannot break it down into a simpler substance. Iron is therefore called an element. Iron atoms, however, are most responsible for iron's behavior. The atoms are unique to iron, and can combine with other atoms of either iron or some other substance.
But if we know what the elements are made up of, what is the atom made up of?
In 1898, J.J. Thompson began experimenting with cathode ray tubes. Cathode ray tubes are comprised of two metal electrodes connected to a battery or a source of electricity.
When an electric field was applied to the cathode ray, Thompson found that the stream of particles bent towards the positive end. He concluded that the stream of particles must be negative. Since he also found that atoms were not charged initially, he guessed that the atoms themselves were made up of negative particles swimming in a sea of positive goop. Think of Thompson's model as the Cocoa Puffs cereal model. The milk in the bowl is like the positive sea in which the negative particles, the Cocoa Puffs, float around.
However, this theory did not last long. In 1911, a mere 89 years ago, a scientist named Ernest Rutherford performed an experiment that changed the face of the atom. Rutherford was interested in uranium. Uranium, as you might know, is unique because it undergoes radioactive decay. In the case of uranium, it spits out positively charged particles known as "alpha particles." We'll cover these later in a section on Half-Life and Radioactive Decay. Rutherford would shoot the particles from the uranium at a piece of gold foil. He wanted to see if the particles would bounce back or if they would go right through the foil. He set up a luminescent screen to detect scattered alpha particles. When he ran the experiment, he found that the majority of the particles went straight through the foil, but some particles that were scattered around the room, and even some that bounced straight back to the detector! Since the alpha particles were positive, Rutherford concluded that the only way the particles could bounce straight back would be if the gold foil also contained positive charges that would repel the alpha particles. If you're a little fuzzy on charges, find your way to the Ions webpage. Rutherford's model of the atom therefore consisted of a positively charged center with an electron floating around it.
