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.


Technetium, symbol Tc, radioactive metallic element, the first element to be created artificially. The atomic number of technetium is 43. Technetium is one of the transition elements of the periodic table.

In 1937 Emilio Segrè and Carlo Perrier created technetium by bombarding molybdenum targets with deuterons (particles consisting of a proton and a neutron). Because technetium is not part of the decay series of any naturally radioactive element (see Radioactivity), scientists had thought that technetium does not occur in nature. In 1988, however, minute quantities of it were detected in ore from a deep molybdenum mine in Colorado. Isotopes ranging in mass number from 90 to 111 are known; the most common isotope has a mass number of 98.

Technetium forms oxides, sulfides, and technetiates, such as ammonium technetiate. Compounds and alloys containing technetium oxide can prevent the corrosion of iron by water. Technetium-99 is used for imaging in medicine. Technetium melts at about 2200° C (about 3992° F), boils at about 4567° C (about 8253° F), and has a specific gravity of 11.5.