What are Dalton's 4 rules for the atom?

Answer 1

Dalton postulated a number of "rules" in his atomic theory, that are now universally accepted as the atomic model, although "rules" might be the wrong word for them. I might be being pedantic on that point though, so I'll leave it. The main points of Dalton's model are:

1) That all elements i.e. Hydrogen, Helium, Oxygen, Nitrogen, Arsenic etc (that is, all substances that can not be broken down further into other substances) are made of tiny particles called "atoms". This was not a new idea. The ancient Greeks, namely Democritus, first postulated the idea that there were "base" substances that all other substances were made out of in varying quantities and proportions. That however is a discussion for another wiki! Currently, there are about 117 known elements with one or 2 others pending confirmation.

2) That all the atoms of any one particular element will be identical to one another. That is, all their physical properties and their composition will not vary, and that if you do encounter atoms that are different from one another, then you are dealing with different elements! So an Oxygen atom will be very different from a Hydrogen or Sulphur atom.

3) Following on from no. 2), given that the atoms of different elements are different from one another, the way to distinguish between them is by their relative atomic weights (determined by the number of protons in the nucleus; in Hydrogen, it's one; Helium - two; Lithium - three; Beryllium - four, and so on).

4) Atoms of different elements can combine together to form "Compounds". A compound of a particular type always contains the same type of elements (and in the same proportions, but we'll come to that later). So Water for example, is a compound that is composed of Hydrogen & Oxygen (as it happens, a single "molecule" (the smallest indivisible unit of a compound) of water contains always and exactly 1 atom of Oxygen and 2 atoms of Hydrogen; hence the formula H2O!)

5) Atoms cannot be created, destroyed, subdivided or changed! Chemical reactions simply reorganise how atoms interact with one another or how they combine with one another. This last postulate is a bit suspect because we now know (thanks to Einstein and a few other clever physicists) that is IS possible to subdivide and/or change an atom (Urnaium-238 can be "changed" into Plutonium-239 by bombarding it with subatomic particles known as Neutrons, and by virtue of the fact that there ARE subatomic particles, we know that you can subdivide them!). But to all intents and purposes the postulation IS correct. Generally speaking an atom of a particular type and hence an element of a particular type don't get altered, "used up" or suddenly pop into existence out of thin air!

Dalton's atomic model is often closely regarded, studied or taught along with the 3 fundamental Laws of matter, one of which he himself postulated. They are:

1) The Law of Conservation of Mass (nothing is created nor destroyed. It changes state or is combined in different ways, but as much of any particular element that you started with, still exists at the end)

2) The Law of Constant Composition (also know as the Law of Definite Proportions, although I think Constant Composition is more explanatory and stops you from confusing it with the 3rd Law (see below). Essentially, that a given compound is made up of particular types of elements, always of the same type and always in the same proportions. No exceptions! If you got variance, then you got a different compound!)

3) The Law of Multiple Proportions (The Exception!... of which chemistry is full of. It's not really an exception though, more of an extension of the previous Law, and sometimes the 2 are confused or not fully understood. Put simply, it simply shows that when you get 2 elements forming 2 or more compounds together, there is always a simple, whole number ratio that describes the composition. There are several Oxides for various elements like Nitrogen, Sulphur or Carbon. Nitrogen Dioxide (NO2) is a different compound from Nitrous Oxide (NO), but contains the same elements, but in different proportions. However, it doesn't matter what quantity or mass you actually have of each of those compounds, the ratio between the elements will always remain the same AND be expressed as a whole number! This is the Law Dalton first expressed in 1803 and, as a result, it's often called "Dalton's Law".

So much of the Atomic model and the 3 Laws are similar or so closely linked that you almost can't have one without the other, hence why the Laws bare mentioning. Dalton was also responsible for the Law of Partial Pressures, which is part of the Gas Laws, but again, that is a discussion for another Wiki!

--Danny Liwszyc--

Answer 2

Dalton's four rules for the atom are: 1) Elements are made up of tiny particles called atoms. 2) Atoms of the same element are identical in size, mass, and other properties. 3) Atoms of different elements can combine to form compounds. 4) Atoms cannot be created or destroyed in chemical reactions, only rearranged.