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.


Hafnium, symbol Hf, metallic element that closely resembles zirconium. Hafnium is one of the transition elements of the periodic table; the atomic number of hafnium is 72.

Hafnium was discovered in Copenhagen in 1923 by the Hungarian chemist Georg von Hevesy and the Dutch physicist Dirk Coster. On the basis of a prediction by the Danish physicist Niels Bohr that element 72 would resemble zirconium in structure, they looked for the element in zirconium ores. Hafnium is found in nearly all ores of zirconium and is 45th in order of abundance of the elements in the crust of the earth. It resembles zirconium so closely in chemical properties and crystal structure that separation of the two elements is extremely difficult. Separation is accomplished most efficiently by means of the ion-exchange technique. Hafnium is used in the manufacture of tungsten filaments. Because of its resistance to high temperatures, it is used with zirconium as a structural material in nuclear power plants.

Hafnium melts at about 2227° C (about 4041° F), boils at about 4602° C (about 8316° F), and has a specific gravity of 13.3. The atomic weight of hafnium is 178.49.