From 1803 to 1808, John Dalton, an English schoolteacher, used the two fundamental laws of chemical combination just described as the basis of an atomic theory. His theory involved three assumptions:
1. Each chemical element is composed of minute, indivisible particles called atoms. Atoms can be neither created nor destroyed during a chemical change.
2. All atoms of an element are alike in mass (weight) and other properties, but the atoms of one element are different from those of all other elements.
3. In each of their compounds, different elements combine in a simple numerical ratio, for example, one atom of A to one of B (AB), or one atom of A to two of
If atoms of an element are indestructible (assumption 1), then the same atoms must be present after a chemical reaction as before. The total mass remains unchanged. Dalton s theory explains the law of conservation of mass.
If all atoms of an element are alike in mass (assumption 2) and if atoms unite in fixed numerical ratios (assumption 3), the percent composition of a compound must have a unique value, regardless of the origin of the sample analyzed.
Dalton s theory also explains the law of constant composition.
Like all good theories, Dalton s atomic theory led to a prediction the law of multiple proportions. B (AB2).
If two elements form more than a single compound, the masses of one element combined with a fixed mass of the second are in the ratio of small whole numbers.
John Dalton (1766 1844), developer of the atomic theory. Dalton has not been considered a particularly good experimenter, perhaps because of his color blindness (a condition sometimes called daltonism).
However, he did skillfully use the data of others in formulating his atomic theory.
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