The Atomic Theory of Matter

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:
  • Each chemical element is made up of very small particles called atoms.
  • All of the atoms of a given element are identical. The atoms of one element are different from the atoms of any other element in a fundamental way. In other words, all atoms of gold (Au) are identical to each other , and all atoms of tin (Sn) are identical to each other, but Au and Sn atoms are different from each other.
  • Atoms combine with each other to form compounds. A given compound always has the same relative numbers of different types of atoms. In other words, all water (H2O) molecules are made up of 2 parts H and 1 part O. All ethanol (C2H5OH) molecules are made up 2 parts C, 6 parts H and 1 part O.
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. (A0.25B1 X 4 = A1B4)  One might expect simpler compounds such as AB, A2B, AB2, or A2B3 to be more probable than A138B207, and this is generally found to be so.


Explanation: The Law of Conservation of Mass

The total number of atoms that react in a chemical reaction does not change. The reaction occurs because the atoms rearrange themselves into new groupings. Therefore we say that 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. Atoms of different elements 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, 100 atoms of hydrogen (of relative mass 1) react with 50 atoms of oxygen (of relative mass 16), one must form 50 molecules of H2O (of relative mass 18). The total mass of the products will always be equal to the total mass of the reactants in any chemical reaction. This makes sense because what you put into a reaction should be what you get out of the reaction if all your doing is rearranging atoms.

                                                        2 H +   1 O =     2 X 1 +   1 X 16 = 18

The same line multiplied 50 times:  100 H + 50 O = 100 X 1 + 50 X 16 = 900/50 = 18 again.


Explanation: The Law of Constant Composition

Atoms are indivisible, and a given atom either is or is not attached to another atom 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 ratios between iron atoms and oxygen atoms - 1:1, 2:3, and 3:4.  In other words all Fe3O4 molecules will be made up of 3 parts Fe and 4 parts O.


Explanation: Law of Multiple Proportions

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, the number of each type of atom in a formula must be an integer. 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).