JOHN DALTON

 John Dalton was trying to find out if atoms of different elements had different masses. He then tried to determine what those masses were. Remember, he did not know the chemical formulas of any of the compounds he was working with at the time.

 The following ratios by weight were found to be correct in lab:

a) 1 gram of hydrogen combines with 8 grams of oxygen to form water

b) 3 grams of carbon combines with 4 grams of oxygen to form carbon monoxide gas

c) 1 gram of hydrogen combines with 3 grams of carbon to form methane gas.

 Dalton reasoned that a 1 to 1 ratio of atoms was the simplest way of combining the elements and if 1 atom of hydrogen combines with 1 atom of oxygen, then oxygen must have a mass 8 times that of hydrogen.

 The weights would thus be: hydrogen = 1, oxygen = 8, carbon = 3

 Dalton knew the ratio of weights was correct but was his assumption of a 1 to 1 ratio correct? He had to find out.

He performed this test to find out. Experiment proved that 3 grams of carbon combines with 8 grams of oxygen to form a gas called carbon dioxide. He reasoned that if carbon monoxide gas had the formula CO, then carbon dioxide must be C0₂ (by looking at the ratio of weights.) This indicated to him that a molecule may contain more than one of a particular kind of atom in some cases.

 Now Dalton realized that he must go back and try to find the correct formula for water (was it HO or something like H₁₇O₄₇ and then figure out the correct weights of atoms.

 Amedeo Avogadro suggested the key, "Equal volumes of different gases contained equal numbers of `particles' (which could be atoms or molecules)". One liter of oxygen gas had, for example, the same number of particles as one liter of any other gas at standard temperature and pressure (here weights do not matter).

 It was found that by splitting water by electrolysis, the volume of hydrogen released was twice that of the volume of oxygen, that is, if you split a certain number of water molecules you get twice as much hydrogen as oxygen (in terms of particles).

Water must then have the formula H₂0. This is better than H0_1/2 which meant splitting atoms, something Democritus said was impossible, until fission of atoms was accomplished with the development of the Atomic Age.

 If 8 grams of oxygen combined with 1 gram of hydrogen, it meant that the single oxygen atom is 8 times as heavy as the two hydrogen atoms taken together. If you still set the atomic weight of hydrogen at 1, then the atomic weight of oxygen is equal to 16 (which keeps the weight ratio of 1 to 8 correct for water).

A test for this theory would go something like this:

 1 liter of hydrogen combines with 1 liter of chlorine to form hydrogen chloride gas. Is this gas made up of 1 atom of hydrogen and 1 atom of chlorine per molecule? Remember that Avogadro said the liter of hydrogen and the liter of chlorine contained equal numbers of particles.

 Logic says that the number of hydrogen chloride molecules formed must only be half as many as the total number of H and Cl atoms that we started with (30 people getting married equals 15 married couples, right) so we should get 1 liter of hydrogen chloride gas.

 1 liter of hydrogen + 1 liter of chlorine = 1 liter of hydrogen chloride gas

 BUT EXPERIMENTS PROVED THAT 1 LITER OF HYDROGEN + 1 LITER OF CHLORINE PRODUCED 2 LITERS OF HYDROGEN CHLORIDE GAS.

 The answer must be that one hydrogen molecule is diatomic (H₂) and combines with one chlorine molecule that is also diatomic (C1₂). This forms 2 molecules of hydrogen chloride gas (HCl + HCl).

 Notice that the hydrogen chloride gas molecule has a one to one ratio of atoms. H₂ + C1₂ --> HCl + HCl (not H₂Cl₂ because then the product would only be 1 liter of hydrogen chloride gas and not the true 2 liters.

 Check: H₂ + 0₂ --> H₂0 which must be balanced to read:

 2H₂ + 0₂ --> 2H₂0 (and the mass ratio of the hydrogen gas to oxygen gas

 4 : 32 (or reduced to 1:8)

 Dalton experimented with other elements and found "elements always combine in the same ratio by weight in making a given compound." This is the LAW OF DEFINITE PROPORTIONS.

 The Law of Definite Proportions occurs only in whole number ratios and it does not matter which mass units are used.