Atomic Theory, the name given to that theory which regards matter as being built up of indivisible particles called atoms, to explain observed chemical facts by assigning certain physical properties to these atoms. The true atomic theory of modern chemistry is due to Dalton and is not yet a century old, but has done a very great deal to forward the science of chemistry, and to procure powerful allies in physics and mathematics. The theory simply states that matter consists ultimately of atoms of different kinds, that atoms combine with other atoms of like or unlike kind forming molecules, and that matter in bulk, such as our senses perceive it, consists of exceedingly large numbers of segregated molecules.
The atom is the smallest quantity of matter that can exist in combination; the molecule is the smallest quantity that can exist alone, and must, therefore, consist of at least one atom. Mercury and zinc give examples of molecules containing only single atoms; hydrogen, oxygen, nitrogen, and most other elements have two atoms; the ozone modifications of oxygen have three; whilst phosphorus and arsenic possess four atoms to the molecule.
If in a quantity of matter all the molecules are alike, the substance is said to be pure; if otherwise, we have a mechanical mixture. If the constituent atoms of the molecules are all alike we have an elementary substance or element. If, while the molecules are alike, they are not composed of like atoms, we have a pure chemical compound. The elements as we know them are not infinite in number, hence the available types of atoms are limited. The properties of all atoms of the same type, i.e. of the same element, are supposed identical throughout the universe, if under the same conditions. One of the most important physical properties of an atom is its mass, from which indeed Mendeleeff's periodic law of the elements enables us to deduce other properties of the substance. The mass of an atom is reckoned in terms of the mass of the hydrogen atom, which is the lightest known to us at present. The atomic weight of an element means therefore the ratio of the weight of its atom to the weight of the hydrogen atom. The following is a table of the atomic weights of the known elements: -
It will now be seen how the following observed laws of chemical combination may be explained: -
(a) The law of fixity of proportions in chemical compounds states that every definite pure substance always possesses the same constitution. Thus water always contains eight-ninths its weight of oxygen, with one-ninth of hydrogen. For on the assumption of the atomic theory, each molecule of water contains two atoms of hydrogen united with one of oxygen. Hence, since the percentage composition of each molecule is a constant, that of any number of molecules will also remain the same.
(b) The law of multiple proportions in chemical compounds states that substances may form different compounds by uniting in fixed proportions, which bear some simple numerical relation to each other. Thus the ratio of the weights of the carbon and oxygen in carbon monoxide are 3:4, in carbon dioxide, 3:8. So also nitrogen and oxygen unite in different proportions, forming a series of oxides whose constituents are in the ratios 14/8, 14/16, 14/24, 14/32, and 14/40. These facts are readily explained. A molecule of carbon monoxide contains one atom of carbon and one atom of oxygen, the ratio of whose weight is 3/4. The molecule of carbon dioxide contains one atom of carbon with two of oxygen; hence the ratio of the constituents is 3/8, Similarly with the nitrogen oxides, we are led to the belief that two atoms of nitrogen unite with one, two, three, four, and five atoms of oxygen, forming these five different kinds of molecules, whose compositions are therefore closely related to each other.
(c) The law of chemical equivalents, chemical quantities which are equal to the same thing as regards their power of doing chemical work or of forming chemical compounds, are equivalent to each other. One gramme of hydrogen will unite with 35.4 grammes of chlorine or with 8 of oxygen. Hence 8 grammes of oxygen are chemically equivalent to 35.4 of chlorine, or two atoms of hydrogen combine with two of chlorine or with one of oxygen; hence two atoms of chlorine are equivalent to one of oxygen, and knowing the respective atomic weights the above numerical relationship may be immediately established.
The next two laws given are not directly deducible from experiment, relating as they do to individual molecules.
(d) Avogadro's Law. - Equal volumes of all gases at the same temperature and pressure contain the same number of molecules, i.e. molecules of all gases under the same conditions of temperature and pressure occupy the same space.
(e) Dulong and Petit's Law: - The atomic weight of an element multiplied by its specific heat is a constant for all elements, known as the atomic heat.
These two laws receive full confirmation from the kinetic theory of gases, as advanced by Clausius, Clerk-Maxwell, and other physicists, and afford the most conclusive means of settling the atomic weight of an element.