1. Models The PMT is one of many models used in science. A model is anything that allows us to better understand a concept. It may be something to look at or touch, but it doesn’t have to be. A model helps us understand how something works. Models may or may not be like the real thing. a globe or a city map.

2. History of the Atom From very early times, scholars called philosophers or thinkers, wondered why matter behaves as it does. Philosophers manipulated matter in their minds, but did no hands-on research or experimentation. According to the Greek four-element theory, all matter consisted of some combination of four elements and everything in the physical world could be explained using these four elements.

4. This view of the elements lasted as late as the early 17 th century since most scholars believed that wisdom came from thinking, not from experimenting. They also believed that only knowledge recorded by ancient philosophers was valid and that this knowledge could not and should not be challenged. There were people around though that believed the four-element theory was not the only view. Hands-on investigations were done by the alchemists.

5. The Alchemists The alchemists were part pharmacist and part mystic. They practiced their craft all over Europe and the Middle East. The base of their belief was hoping to change base elements, such as lead, into gold. Alchemists developed many useful procedures, such as distillation, to probe the properties of many materials. They were very secretive with their findings and even developed secret codes so that nobody else could steal their work.

6. John Dalton John Dalton introduced a new way of explaining chemical facts and laws in his Atomic Theory.

7. Dalton’s Atomic Theory 1.All matter is composed of small indivisible particles called atoms. 2.Atoms cannot be created, destroyed, or divided into smaller particles. 3.All atoms of the same element are identical in mass and size. 4.All atoms of different elements differ from each other in size and mass. 5.Compounds are created when atoms of different elements combine in definite proportions.

8. Dalton’s atomic theory is based on a scientific model that is different from the model used in the particle theory. Dalton’s model uses the idea that elements are different because of their particles. The particle theory is based on particles being identical. Dalton’s model lead to a more accurate definition of an element.

9. Element An elements is a pure substance made up of one type of particle (atom). Each element has its own distinct properties and cannot be broken down into simpler substances by means of chemical change.

10. Laws and Theories A scientific law does not explain anything. A law describes and summarizes what happens. Theories are imaginative ways to explain why something happens. A description is different than an explanation.

11. Dalton published a book that provided the relative masses for 20 elements. He also showed 17 examples of how the theory explained the formation of a compound. Dalton’s Atomic Theory gave chemists a great deal to think about. It provided a quantitative basis for thinking about the structure of matter. It also stimulated a lot of discussion and experimentation, enabling scientists to explain the structure and behavior of elements and compounds.

12. Phosphorus was the only element discovered in the 17 th century. Its discovery brought the number of known elements up to 14. In the 18 th century, the pace of discovery quickened. Discovering a new element was a sure path to scientific fame! 18 more elements were identified by 1800. Chemical research was so competitive that some over eager chemists neglected safety precautions and dies while conducting experiments.

13. 50 more elements were discovered in the 19 th century. Interpreting this rapidly growing body of knowledge was difficult though. Most of the new elements were metals with similar properties – shiny, silvery-grey, good conductors of heat and electricity. It wasn’t always easy to prove new elements.