Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Rubidium (Latin rubidus, “red”), symbol Rb, chemically reactive metallic element with an atomic number of 37. In group 1 (or Ia) of the periodic table, rubidium is one of the alkali metals.

Rubidium was discovered spectroscopically in 1860 by the German chemist Robert Wilhelm Bunsen and the German physicist Gustav Robert Kirchhoff, who named the element after the red lines prominent in its spectrum. Metallic rubidium is silvery white and very soft. After cesium, it is the most active of the alkali metals. It tarnishes immediately upon exposure to air and ignites spontaneously to form rubidium oxide. It reacts violently with water. In general chemical behavior, rubidium resembles sodium and potassium. Rubidium melts at about 39° C (about 102° F), boils at about 686° C (1267° F), and has a specific gravity of 1.53; the atomic weight of rubidium is 85.468.

It is a widely distributed element, ranking 16th in order of abundance of the elements in the crust of the earth. It is not found in large deposits but occurs in small amounts in certain mineral waters and in many minerals usually associated with other alkali metals. It is also found in small quantities in tea, coffee, tobacco, and other plants, and trace quantities of the element may be required by living organisms. Rubidium is used in making certain catalysts and in photoelectric cells. The rate of radioactive decay of the isotope rubidium-87 can be used in geologic age determination (see Radioactivity).