John Dalton and the Atomic Theory

John Dalton by Charles Turner after James Lonsdale (1834, mezzotint)

In 1803, John Dalton, proposed his atomic theory. This was more than a statement that atoms existed, it was an explanation of how and why matter behaved as it did. Dalton stated the following:

1. Matter is made of tiny, indivisible particles called atoms
2. All atoms of the same element are the same; atoms of different elements are different
3. Atoms combine in whole number ratios

Notice that this set of statements can be used to define (for the first time) what a chemical reaction is -- a rearrangement of atoms.

Where did these ideas come from?

Chemistry, at the beginning of the 19th century, was still emerging from alchemy--the attempt to turn "base" metals into gold and to produce the philosopher's stone.

Two laws were developed during the course of this work--the Law of Definite Proportions (also called the Law of Constant Composition) and the Law of Multiple Proportions

The Law of Definite Proportions (also called the Law of Constant Composition) states that a compound is always made of the same elements in the same proportion. In simple terms, water is always composed of 1 g of hydrogen for every 8 grams of oxygen.

The Law of Multiple Proportions is a bit muddier, but also very useful. It states that when two compounds are made of the same two elements, the amount of the first element that mixes with 1 g of the second element in each of the two compounds will form a simple ratio.

More details about these laws, along with an analogy to help make sense of them, can be found here.

Dalton's Theory can thus be understood as a way to make sense out of these ideas and Lavoisier's Law of Conservation of Mass.

• Molecules are combinations of atoms and the combinations are always the same (Definite Proportions). 
• Different compounds have different numbers of atoms, but since two atoms must weigh exactly twice one atom, there will be a simple ratio between those masses (Multiple Proportions).
• Reactions are just a rearrangement of the atoms (Conservation of Mass).

Nobody's Perfect

There are, of course, a few flaws in Dalton's statements.

We discovered, later, that atoms are not indivisible, and we discovered isotopes (atoms of the same element that are not identical). However, it can be stated that an atom is the smallest piece of any element that maintains the properties of that element and that isotopes are chemically identical (they differ in mass). So, in the end, not to bad for 1803!