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" . 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:
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?