AT_Dalton's Atomic Theory Lesson

Dalton's Atomic Theory

The concept of the atom began nearly 2500 year ago by Greek philosophers. They believed that matter was made up of tiny, indivisible particles. The word atom comes from the Greek word "atomos" which means "not cut". At this point in history, the atom was just an idea, a concept not based on any observation or scientific data.

A theory must explain what has been observed and also be able to make correct predictions. The first atomic theory to follow these rules was developed by a English scientist named John Dalton

There were two scientific laws that were the experimental foundation for Dalton's atomic theory.

1. When a compound is formed elements always combine in the same mass proportion. This became known as the Law of Definite Proportions. For example, when water is decomposed into its elements, the ratio of mass of oxygen to mass of hydrogen is always 8:1. Notice this example talks about mass, NOT the number of elements.

2. When a reaction takes place in a closed vessel, so that nothing can enter or escape, the total mass after the reaction takes place is always the same as it was before the reaction. This observation was made over large numbers of chemical reactions with always the same result. This became what we now know as the Law of Conservation of Mass

Dalton, and other scientists, asked themselves what must matter be made of, knowing these laws to be true.

In the early 1800's, Dalton proposed his atomic theory. It stated:

1. Matter is made of tiny particles called atoms.
2. Atoms of the same element are identical in size, mass, and other properties.
3. Atoms of different elements are different in size, mass, and other properties.
4. When atoms of different elements combine to form compounds, they do so in a fixed numerical ratio.
5. Atoms are indivisible. They can rearrange, but do not themselves break apart.

Law of Multiple Proportions

A big success of Dalton's atomic theory was that it predicted another law, the Law of Multiple Proportions. Remember, being able to correctly predict is the mark of a good theory!

Whenever two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers.

Before you watch the video below, you need to make sure that you remember how to count atoms within chemical formulas. Do the following to check your knowledge.

Remember to work on the module practice problems as you complete each section of content.

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