Early in the nineteenth
century, John Dalton put forth the first useful theory describing matter as consisting
of
atoms. Dalton's atomic theory can be summarized in the following four
postulates:
The law of constant composition
says that any given compound always consists of the same atoms and the same
ratio of atoms. For example, water always consists of oxygen and hydrogen atoms,
and it is always 89 percent oxygen by mass and 11 percent hydrogen by mass.
The law of conservation
of mass says that the total mass of materials before and after a chemical reaction
must be the same. For example, if we combine 89 grams of oxygen with 11 grams
of hydrogen under the appropriate conditions, 100 grams of water will be produced—no
more and no less.
In addition to explaining
these two well-known laws, Dalton used his atomic theory to predict another
law. The law of multiple proportions,
which states that if two elements combine to form more than one compound,
the masses of one of the elements that can combine with a given mass of the
other element are related by factors of small whole numbers. For example, water
has an oxygen-to-hydrogen mass ratio of 7.9:1. Hydrogen peroxide, another compound
consisting of oxygen and hydrogen, has an oxygen-to-hydrogen mass ratio of 15.8:1.
The ratio of these two ratios gives a small whole number.
Multiple
Proportions
Historical Development
law of constant composition was actually known as the law of definite proportions and stated that the elemental composition
of a pure compound is always the same, regardless of its source. It was proposed as and axiom of Chemistry by a French scientist named Proust . It was in direct opposition to the previous view that elements could combine to form an almost infinite number of compounds, the product being determined by relative concentration and chemical affinity. It preceeded Dalton's atomic theory, though he would reformulate it using the new terminology.
The law of conservation of mass was a truly seminal concept proposed by A. Lavoisier in 1789 (guillotined in 1794 as a nobleman and tax-collector) that laid the foundation for modern chemistry and sounded the deathknell for the Phlogiston Theory . Phlogiston is a vitalist view of chemistry, an imponderable fluid that is given off by substances (e.g. charcoal) rich in phlogiston burn to liberate phlogiston. In certain instances phlogiston could pass from one substance to another, for example in smelting where phlogiston pass from the charcoal to the ore, leaving behind a formless pile of ashes.
The law of multiple proportions was formulated as the "rule of greatest simplicity" and resulted in his assumption that water was HO, with oxygen having a relative mass of 12.
Dalton was actually a very poor experimentalist and "tolerated a very generous margin of error.
The atomic hypothesis first appears on page 213 of a book titled New System of Chemical Philosophy and by modern standards has some glaring errors
The Atomic Theory of Matter
