Before we can discuss Dalton's Model of the atom, I must first mention the Law of Multiple Proportions. Simply put, when elements combine to form compounds they do so in whole number ratios.
Carbon and Oxygen can form either CO ( a 1:1 ratio) or CO2 (a 1:2 ratio). It also states that if the mass of the first element is constant the second elements masses would be in a whole number ratio.
1.00g of Carbon will have combined with 1.33g in CO
1.00g of Carbon will have combined with 2.66g in CO2
The mass are in a 1:2 ratio
Simply put if combinations are in whole number ratios, there must be a one. This one being an atom. Here are Dalton's assumptions.
All matter is composed of atoms