Dalton's idea of an element is what we believe today - an element is a chemical substance that cannot be decomposed further by chemical means (i.e. heat, electricity, reacting with another chemical).
Daltonian atoms are usually taught as being similar to featureless billiard balls. In truth, Dalton never ruled out the possibility of subatomic structure. He just knew that the state of the art in the early 1800's did not allow the physical structure of an atom to be probed.
2.The atoms of an element are identical in their masses
Atoms of the same element have the same properties, such as weight. Atoms of different elements have different properties, including a different weight. The idea that all atoms of a given chemical element weigh the same is known today to be incorrect.
Also, the concept of chemical combination in 1803 was much, much different than what Dalton was proposing. Although Dalton was well-known at the time, the most authoritative chemist of the period was Claude Louis Berthollet.
Even many years later, Berthollet resisted the idea of the atom: that elements combine in small, whole number ratios that are fixed. Even into the late 1800's, there were French chemists who used their authority to punish lesser colleagues and students who publically supported the chemical atomic theory and he was the first, in 1798, to observe a reversible reaction.
3. Atoms of different elements have different masses
Although this idea is implicit in Dalton's theory, it is not original with him. Once again, the Greeks. developed this general idea. This idea was even discussed in chemical textbooks of Dalton's time.
However, here is where we meet the original contribution of Dalton. Arnold W. Thackray says (his original is in italics):
"The particular development of Dalton, which distinguishes his chemical atomic theory from earlier work, was his devising of an effective system to obtain these relative particle weights from currently available chemical data. . . ."
This means that while it was claimed atoms of different elements had different weights, no one could figure out what the different weight values were. Dalton was the first to do so.
4. Atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on.
Chemical combination between two or more atoms occur in simple, numerical ratios (i.e., 1 to 1, 1 to 2; 2 to 3; etc.).
This point gives immediate explanation to the Law of Definite Proportions, announced by Joseph Louis Proust in 1797.
During his research, Dalton discovered the Law of Multiple Proportions, another law, which is easily explained by his atomic theory.
Dalton discovered this law while studying some of the oxides of nitrogen. The law is:
Atoms of the same element can unite in more than one ratio with another element to form more than one compound.
5. Atoms can be neither created nor destroyed
A fifth idea implied in Dalton's theory, but usually not discussed is this: atoms can be neither created nor destroyed. An element's atoms do not change into other element's atoms by chemical reactions. For example, nitrogen and oxygen atoms stay as they even when combined. They can be recovered by decomposing the substance.
