RADIUM (Symbol Ra, atomic number 88, atomic weight 225-95) is the best known and the most important member of the radio-elements, discovered by Professor and Mme. Pierre Curie and G. Bemont of Paris in 1898. The position of radium in the periodic system of classification is unmistakably that of the heaviest member of the alkaline-earth metals, calcium, strontium and barium, and its chemical properties and all its physical properties, except the most important one, its radioactivity, are those to be expected from the element which occupies this posi tion. It is as a radioactive substance, however, and not merely as a chemical element, that radium claims attention, and as such it is dealt with in the article RADIOACTIVITY.
The rate at which disintegration occurs proceeds always accord ing to one fixed plan. In a given interval of time, a second or a day or a year, a definite fraction of all the atoms which make up the radio-element, breaks up, and this fraction, fixed by Nature, cannot be varied by man ; it is the same whether millions of millions of atoms are being considered or a few thousands only. No chemical combination of the radio-element with other atoms, no physical agency such as enormous temperatures or pressures seems able to affect the value of this fraction at all. For radium this fraction is per year : this means that if we were to weigh out 2,28o units of this element to-day, in a year's time 2,279 would be quite unchanged and I would have been broken up. The radium that has broken up, however, is not simply radium
in some other form, it is something entirely different; the expul sion of either an a- or a /3-particle has so altered the radium atom that it has become a new one. A radio-element, such as radium, consequently contains at least two kinds of atoms, those that have already broken up and those that have not. The unchanged atoms comprise what is called the parent substance ; the residual atoms compose the product. This product is perfectly distinct from its parent in physical and chemical properties, and can be easily separated from it by the ordinary methods of analytical chemistry. Radium's product is the gaseous element radon (q.v.), which is the heaviest of the inert gases. Radium being a solid and radon a gas at the ordinary temperature, their separation is an obvious one. If now the product happens, like its parent, to be radioactive, a certain fraction of it will disintegrate in a unit of time to form a third substance, and this substance, if radio active, will produce a fourth, the fourth a fifth, and so on, till a substance is reached which is not radioactive; whereon this series of radio-elements ends abruptly. Such a series is called a disinte gration series; in it each element is the parent of the one that follows and the product of the one that precedes. Radium and radon are respectively the sixth and seventh of the disintegration series that begins with the rare element uranium and ends with the common element lead. (See RADIOACTIVITY.) In the accompanying table are set out the elements of the ura nium-radium-lead disintegration series, with the symbols by which they are described in the literature on radioactivity, the nature of the particle or ray expelled by the element, the atomic weights of the elements, and finally their atomic numbers. It will be seen from the table that when an a-particle is expelled, the atomic weight of the product is 4 units less and the atomic number two less than that of the parent ; and that when a 0-particle is ex pelled the atomic weight of the product is identical with that of the parent, but the atomic number is one greater. The reasons for this generalization are given under RADIOACTIVITY.