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It is always mentioned that Dalton proposed an Atomic Theory. But when you read it, it consists of the following:

The main points of Dalton’s atomic theory, an explanation of the structure of matter in terms of different combinations of very small particles, are given by the following postulates:

  1. All matter is composed of indivisible atoms. An atom is an extremely small particle of matter that retains its identity during chemical reactions.

  2. An element is a type of matter composed of only one kind of atom, each atom of a given kind having the same properties. Mass is one such property. Thus, the atoms of a given element have a characteristic mass.

  3. A compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions. The relative numbers of any two kinds of atoms in a compound occur in simple ratios. Water, for example, a compound of the elements hydrogen and oxygen, consists of hydrogen and oxygen atoms in >the ratio of 2 to 1.

  4. A chemical reaction consists of the rearrangement of the atoms present in the reacting substances to give new chemical combinations present in the substances formed by the reaction. Atoms are not created, destroyed, or broken into smaller particles by any chemical reaction.

(From the book: General Chemistry by Ebbing and Gammon)

I'm sure many of you might be knowing that postulates and theories are quite different. Can someone explain me whether Dalton proposed a postulate or a theory?

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It is most definitely a theory. The 4 points you mention could be construed as hypothesis by themselves. But Dalton went further, he put them to the test. The results of his exeriments formed yet another component of his atomic theory called the Law of Multiple Proportions. This law served as proof that the other components of his theory were true. I used to spend most of a class period explaining this law to students, it's best you search the web for a complete explaination but I'l give you a brief summary via example.

Say an experiment shows it takes exactly 16 g of oxygen to combine with 12 g of carbon to make carbon monoxide. Now suppose we make carbon dioxide. We would find that it now takes 32 g of oxygen to combine with that same amount, 12 g, of carbon. So, for a fixed amount of carbon, the amount of oxygen required to make the two different compounds can be expressed as a small, whole number ratio. What would explain this better than to surmise that elements come as particles and thes particles combine in small, whole number ratios to form compounds.

When a hypothesis have been proven by repeatable experimentation, we call it a theory.

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