We have come to the work of John Dalton who offered an atomic theory of elements right around the start of the 1800’s.  He proposed that each element was a species of atom of a distinctive size and weight.  What made his approach so compelling was his ability to push the argument to get at chemical recipes, like H₂O or CO₂.…a truly remarkable advance.  Let’s see how he did it.

Dalton’s work grew from several reasonable hypotheses:

• That any atom of a given element is like any other
• That any molecule of a given compound is like any other
• That stable compounds like water or ammonia are clusters of atoms with simple combining ratios, like 1 to 1 or 2 to 1…nothing exotic, like 17 to 3.

These seem pretty straightforward.  How could they lead to chemical equations?

We can take the case of water.  Scientists had already established that water was made from oxygen and hydrogen and that 16 grams of oxygen plus 2 grams of hydrogen would give you 18 grams of water.  If we take the chemical formula for water to be HO, then oxygen atoms are 8 times heavier than hydrogen atoms!  Similarly, ammonia is made of nitrogen and hydrogen. 14 grams of nitrogen plus 3 grams of hydrogen yield 17 grams of ammonia.  Taking the chemical formula to be HN, Dalton gets a relative atomic weight for nitrogen as 5 times the weight of hydrogen.  In his text, A New System of Chemical Philosophy, published in 1808, Dalton offers the relative atomic weights of 20 elements.

This was really exciting; but at the same time, Dalton’s chemical formulas for water and ammonia were ‘wrong’.  Water is H₂0 and ammonia is NH₃.  How these came to be corrected is an important story.