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Bloop 10-09-2012 06:17 PM

Science 9 difficulties - Daltons atomic theory
 
Okay, so this is grade 9 science and it isn't insanely hard, but I'm doing some research for a project in which I have to explain the history of the atomic theory. Besides the five main points, theres one extra point that just doesn't make sense. Here is what it says:

When atoms combine in only one ratio, "..it must be presumed to be a binary one, unless some cause appear to the contrary".

I'm really just having difficulty understanding what this means. Could somebody explain what this means? Thanks.

tiny 10-09-2012 06:36 PM

John Dalton - Wikipedia, the free encyclopedia

towerofcards.19 10-09-2012 06:38 PM

That's the one point that a lot of his peers and colleagues didn't agree on, and the one point that was real controversial.

Dalton is stating that when atoms combine to form molecules, the ratio between the different atoms(elements) is one to one. Thus he believed water was OH instead of H2O. I don't really remember much but that's the gist of it. This final point was drawn from his previous points, even though Dalton had no evidence for it. I do know, however, that as of present day, at least four of his points still hold.

And Wikipedia isn't a reliable source for research...

tiny 10-09-2012 06:46 PM

Quote:

Originally Posted by towerofcards.19 (Post 2499409)

And Wikipedia isn't a reliable source for research...

Only if you're a second year research methods professor over the age of 40.

*Self_DeFenCe* 10-09-2012 07:31 PM

Quote:

Originally Posted by tiny (Post 2499412)
Only if you're a second year research methods professor over the age of 40.

So true....

Bloop 10-09-2012 07:48 PM

Thanks for the help, makes a lot more sense now :D

Talfuchre 10-09-2012 08:10 PM

Are you sure Tower? I thought it was a fixed ratio - not one to one -

" In 1803 Dalton noted that oxygen and carbon combined to make two compounds. Of course, each had its own particular weight ratio of oxygen to carbon (1.33:1 and 2.66:1), but also, for the same amount of carbon, one had exactly twice as much oxygen as the other. This led him to propose the Law of Simple Multiple Proportions, which was later verified by the Swedish chemist Berzelius. In an attempt to explain how and why elements would combine with one another in fixed ratios and sometimes also in multiples of those ratios, Dalton formulated his atomic theory.

The idea of atoms had been proposed much earlier. The ancient Greek philosophers had talked about atoms, but Dalton's theory was different in that it had the weight of careful chemical measurements behind it. It wasn't just a philosophical statement that there are atoms because there must be atoms. His atomic theory, stated that elements consisted of tiny particles called atoms. He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass. He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses. He also said that compounds consisted of atoms of different elements combined together. Compounds are pure substances (remember they cannot be separated into elements by phase changes) because the atoms of different elements are bonded to one another somehow, perhaps by hooks, and are not easily separated from one another. Compounds have constant composition because they contain a fixed ratio of atoms and each atom has its own characteristic weight, thus fixing the weight ratio of one element to the other. In addition he said that chemical reactions involved the rearrangement of combinations of those atoms."

towerofcards.19 10-09-2012 09:02 PM

Talfuchre, from what I understand, Dalton's atomic theory was the attempt at explaining fixed ratios, and from that theory, he came up with the Law of Multiple Proportions (multiple fixed ratios) like you put above.

However, when Dalton stated "When atoms combine in only one ratio, '..it must be presumed to be a binary one, unless some cause appear to the contrary'." that was his 'rule of greatest simplicity', which was different from the LoMP. Dalton assumed that if two elements bond to form a compound, then assume that the ratio between the two element atoms is one to one, because that's the 'simplest' combination there is. Of course this is wrong as proven by modern science, but that was Dalton's assumption, and he knew it wasn't necessarily correct.

Mr.Smith 10-09-2012 10:34 PM

Plagerised from here:

"Using his theory, Dalton rationalised the various laws of chemical combination which were in existence at that time. However, he made a mistake in assuming that the simplest compound of two elements must be binary, formed from atoms of each element in a 1:1 ratio, and his system of atomic weights was not very accurate - he gave oxygen an atomic weight of seven instead of eight.
Despite these errors, Dalton's theory provided a logical explanation of concepts, and led the way into new fields of experimentation."

This explains it even better:

When elements react, their atoms sometimes combine in more than one simple, whole-number ratio. Dalton used this postulate to explain why the weight ratios of nitrogen to oxygen in various nitrogen oxides were themselves simple multiples of each other. Even Dalton's critics were impressed by the power and simplicity of his explanation, and it persuaded many of them that his atomic theory was worthy of further investigation.
Unfortunately, Dalton included an additional postulate that prevented his theory from being accepted for many years. When atoms combine in only one ratio, Dalton said, "..it must be presumed to be a binary one, unless some cause appear to the contrary" [2]. He had no experimental evidence to support this postulate, and it lead him to mistakenly assume that the formula of water was OH and the formula of ammonia was NH. As a result, Dalton's atomic weights for oxygen and nitrogen were incorrect and his experimental data did not support many of the conclusions he drew from it.

A consistent set of atomic weights was absolutely essential before the theory could be accepted and applied. Next, we'll see how Dalton's postulates can be used to estimate atomic weights from experimental data, and how they explain three basic laws of chemistry.



Basically what I got out of this thread + those two website + taking that course last year, it that Dalton said elements combine in definite or fixed ratios, but he went a step further saying (wrongly!) that they had to combine 1:1. Obviously that's not true, water has two hydrogen and one oxygen.

If you really want to impress your teacher explain that he created the "Law Of Multiple Proportions". Wikipedia explains it best here:

"For example, Dalton knew that the element carbon forms two oxides by combining with oxygen in different proportions. A fixed mass of carbon, say 100 grams, may react with 133 grams of oxygen to produce one oxide, or with 266 grams of oxygen to produce the other. The ratio of the masses of oxygen that can react with 100 grams of carbon is 266:133 ≈ 2:1, a ratio of small whole numbers. Dalton interpreted this result in his atomic theory by proposing (correctly in this case) that the two oxides have one and two oxygen atoms respectively for each carbon atom. In modern notation the first is CO (carbon monoxide) and the second is CO2 (carbon dioxide)."

That is really the best way I think anyone can explain it, right there.^

EDIT: To explain this further and a little more visually:
http://upload.wikimedia.org/wikipedi...roportions.jpg

Note the data for NO, NO2 and NO4. The oxygen atoms combine at a fixed ratio, and therefore each atom adds a fixed weight. NO2 has double the mass of oxygen of NO, and NO4 has quadruple the mass of oxygen of NO.

Does that make more sense?

Bloop 10-09-2012 10:35 PM

Thanks guys for the help :D


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