James Richard Fromm
The understanding of the nature of matter which is called the atomic theory of matter, first postulated by John Dalton, is the basis of all modern chemistry. The observations which Dalton's theory explains had in many cases been made long before his formulation of the theory, and it was these observations and empirical relationships which led him to it. As stated by Dalton, the atomic theory of matter consists of three postulates:
The atomic theory of matter holds that atoms are the fundamental units of matter and that atoms are conserved in chemical reactions. In other words, chemical reactions consist of rearrangements of atoms to form compounds while the atoms themselves remain unchanged. Then only certain compounds can exist; the molecule AB is possible while the molecule A0.25B is not, although the molecule AB4 would have the same stoichiometry and would exist. One might expect simpler compounds such as AB, A2B, AB2, or A2B3 to be more probable than more complex compounds like A256B413, and this is generally found to be so.
The law of conservation of total mass is a consequence of the atomic theory of matter. Since the numbers of atoms are not changed in any chemical reaction, or in other words a chemical reaction is the rearrangement of atoms to form different molecules, atoms are conserved both in number and in type whenever a chemical reaction occurs.
Dalton's second postulate was that all atoms of the same element are identical and differ from the atoms of any other element in some fundamental way. Dalton knew that one of the ways in which the atoms of one element differ from those of another is mass. If, for example, 136 atoms of hydrogen (of relative mass 1) react with 68 atoms of oxygen (of relative mass 16), one must form 68 molecules of H2O (of relative mass 18). As a consequence, the total mass of the products will always be equal to the total mass of the reactants in any chemical reaction.
The law of constant composition of compounds is a consequence of the atomic theory of matter. Atoms are indivisible, and a given atom either is or is not attached to another to form a compound. The empirical formula of a compound is the ratio of atoms of one type to atoms of another in the compound. The three oxides of iron, which have the empirical formulae FeO, Fe2O3, and Fe3O4, correspond to three different stoichiometric ratios between iron atoms and oxygen atoms - 1:1, 2:3, and 3:4. Iron oxides of other stoichiometries, such as 3:2, would not be forbidden by atomic theory but have never been found.
Empirical formulae such as Fe2O3 can be used interchangeably with molecular formulae in using the law of constant composition. For the oxides of iron and for many solid compounds, only empirical formulae are meaningful because the compounds do not exist as isolatable molecules but only in a stoichiometric crystal lattice. For molecular benzene, C6H6, the empirical formula CH could be used but the molecular formula is preferred. Chemists always use the molecular formula, if there is one, in preference to an empirical formula.
The law of multiple proportions of elements in compounds is a consequence of the atomic theory of matter. The compounds CuO and Cu2O have compositions in which the ratios of the elements are 1:1 and 2:1 respectively. Since atoms are indivisible, a compound can have only integral numbers of atoms of each type in its molecules.
Example. Water (molecular mass 18) has one atom of oxygen (atomic mass 16) to two atoms of hydrogen (atomic mass 1). The mass ratio of hydrogen to oxygen in water is 2/16 = 0.125. Hydrogen peroxide (molecular mass 34) has two atoms of oxygen (atomic mass 16) to two atoms of hydrogen (atomic mass 1), so the mass ratio of hydrogen to oxygen in hydrogen peroxide is 2/32 = 0.0625. The mass ratio of hydrogen to oxygen in water (H2O) is exactly twice the mass ratio of hydrogen to oxygen in hydrogen peroxide (H2O2).