# Dalton's Atomic Theory

### Learning Objectives

List the components of Dalton’s atomic theory.

### How do we experiment?

Pick a little, talk a little, pick a little, talk a little,

Cheep cheep cheep, talk a lot, pick a little more

These lyrics from the musical “Music Man” summed up the way science was done for centuries. OK, the lyrics referred to a group of gossiping ladies, but the outcome was the same. The Greek and Roman philosophers debated, discussed, and sometimes even attacked one another. But the mode of discovery was talk. There was no experimentation—the idea had not been thought of yet. So science did not develop very far and there was no reliable way to establish what was true and what was false.

## John Dalton

Figure 1. Dalton.

While it must be assumed that many more scientists, philosophers and others studied the composition of matter after Democritus, a major leap forward in our understanding of the composition of matter took place in the 1800s with the work of the British scientist John Dalton. He started teaching school at age twelve, and was primarily known as a teacher. In his twenties, he moved to the growing city of Manchester, where he was able to pursue some scientific studies. His work in several areas of science brought him a number of honors. When he died, over 40,000 people in Manchester marched at his funeral.

Dalton studied the weights of various elements and compounds. He noticed that matter always combined in fixed ratios based on weight, or volume in the case of gases. Chemical compounds always contain the same proportion of elements by mass, regardless of amount, which provided further support for Proust’s law of definite proportions.  Dalton also observed that there could be more than one combination of two elements.

### Dalton’s Atomic Theory (1804)

From his experiments and observations, as well as the work from peers of his time, Dalton proposed a new theory of the atom. This later became known as Dalton’s atomic theory. The general tenets of this theory were as follows:

• All matter is composed of extremely small particles called atoms.
• Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties.
• Atoms cannot be subdivided, created, or destroyed.
• Atoms of different elements can combine in simple whole number ratios to form chemical compounds.
• In chemical reactions, atoms are combined, separated, or rearranged

Dalton’s atomic theory has been largely accepted by the scientific community, with the exception of three changes. We know now that (1) an atom can be further sub-divided, (2) all atoms of an element are not identical in mass, and (3) using nuclear fission and fusion techniques, we can create or destroy atoms by changing them into other atoms.

Figure 2. Dalton's symbols.

### Summary

• Dalton proposed his atomic theory in 1804.
• The general tenets of this theory were as follows:

• All matter is composed of extremely small particles called atoms.
• Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties.
• Atoms cannot be subdivided, created, or destroyed.
• Atoms of different elements can combine in simple whole number ratios to form chemical compounds.
• In chemical reactions, atoms are combined, separated, or rearranged

### Practice

Use the link below to do the exercise.  Read the sections and take the quiz at the end.

http://antoine.frostburg.edu/chem/senese/101/atoms/dalton.shtml

### Review

1. How did the Greek and Roman philosophers study nature?
2. When did John Dalton start teaching school?
3. Did Dalton believe that atoms could be created or destroyed?
4. List the basic components of Dalton’s atomic theory.
5. What parts of the theory are not considered valid any more?

## Glossary

• atom:   The smallest unit of an element.
• atomic theory:  The general tenets of Dalton's atomic theory were as follows:

• All matter is composed of extremely small particles called atoms.
• Atoms of a given element are identical in size, mass, and other properties.
• Atoms of different elements differ in size, mass, and other properties.
• Atoms cannot be subdivided, created, or destroyed.
• Atoms of different elements can combine in simple whole number ratios to form chemical compounds.
• In chemical reactions, atoms are combined, separated, or rearranged