differ across these compounds by ratios of small whole numbers. Dalton concluded from all this that elements react with each other in discrete and consistent units of weight. Dalton decided to call these units "atoms". Example 1 — tin oxides: Dalton identified two oxides of tin. One is a grey powder (which Dalton referred to as the "protoxide") in which for every 100 parts of tin there is 13.5 parts of oxygen. The other oxide is a white powder (which Dalton referred to as the "deutoxide") in which for every 100 parts of tin there is 27 parts of oxygen. 13.5 and 27 form a ratio of 1:2. Dalton concluded that in the grey oxide, there is one oxygen atom for every tin atom, and in the white oxide there is two oxygen atoms for every tin atom. These oxides are today known as tin(II) oxide (SnO) and tin(IV) oxide (SnO2) respectively. Example 2 — iron oxides: Dalton identified two oxides of iron. One is a black powder in which for every 100 parts of iron there is about 28 parts of oxygen. The other is a red powder in which for every 100 parts of iron there is 42 parts of oxygen. 28 and 42 form a ratio of 2:3. These oxides are today known as iron(II) oxide (better known as wüstite) and iron(III) oxide (the major constituent of rust). Their formulas are Fe2O2 and Fe2O3 respectively. Example 3 — nitrogen oxides: Dalton mentioned three oxides of nitrogen: "nitrous oxide", "nitrous gas", and "nitric acid" (these compounds are known today as nitrous oxide, nitric oxide, and nitrogen dioxide respectively). "Nitrous oxide" is 63.3% nitrogen and 36.7% oxygen, which means it has 80 g of oxygen for every 140 g of nitrogen. "Nitrous gas" is 44.05% nitrogen and 55.95% oxygen, which means there is 160 g of oxygen for every 140 g of nitrogen. "Nitric acid" is 29.5% nitrogen and 70.5% oxygen, which means it has 320 g of oxygen for every 140 g of nitrogen. 80 g, 160 g, and 320 g form a ratio of 1:2:4. The formulas for these compounds are N2O, NO, and NO2. Determining the atomic weights The atomic weight of an element measures how heavy an atom of that element is compared to atoms of the other elements. Dalton and his contemporaries could not measure the absolute weight of atoms—i.e. their weight in grams—because