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.


Helium (Greek helios, “sun”), symbol He, inert, colorless, odorless gaseous element. In group 18 (or VIIIa) of the periodic table, helium is one of the noble gases. The atomic number of helium is 2.

The French astronomer Pierre Janssen discovered helium in the spectrum of the corona of the sun during an eclipse in 1868. Shortly afterward it was identified as an element and named by the British chemist Sir Edward Frankland and the British astronomer Sir Joseph Norman Lockyer. The gas was first isolated from terrestrial sources in 1895 by the British chemist Sir William Ramsay, who discovered it in cleveite, a uranium-bearing mineral. In 1907 the British physicist Sir Ernest Rutherford showed that alpha particles
are the nuclei of helium atoms, which later investigation confirmed.

Helium has monatomic molecules, and is the lightest of all gases except hydrogen. Helium solidifies at -272.2° C (-457.9° F) at pressures above 19,000 torr (25 atmospheres); helium boils at -268.9° C (-452.0° F) and has a density of 0.1664 g/liter at 20° C (68° F). The atomic weight of helium is 4.0026.

Helium, like the other noble gases, is chemically inert. Its single electron shell is filled, making possible reactions with other elements extremely difficult and the resulting compounds quite unstable. Molecules of compounds with neon, another noble gas, and with hydrogen have been detected, however, and other compounds have been suggested. Because of helium's abundance in the universe, the existence of such reactions, however rare, could be of importance in cosmology.

Because it is noncombustible, helium is preferred to hydrogen as the lifting gas in lighter-than-air balloons; it has 92 percent of the lifting power of hydrogen, although it weighs twice as much. Helium is used to pressurize and stiffen the structure of rockets before takeoff and to pressurize the tanks of liquid hydrogen or other fuel in order to force fuel into the rocket engines. It is useful for this application because it remains a gas even at the low temperature of liquid hydrogen. A potential use of helium is as a heat-transfer medium in nuclear reactors because it remains chemically inert and nonradioactive under the conditions that exist within the reactors.

Helium is used in inert-gas arc welding for light metals such as aluminum and magnesium alloys that might otherwise oxidize; the helium protects heated parts from attack by air. Helium is used in place of nitrogen as part of the synthetic atmosphere breathed by deep-sea divers, caisson workers, and others, because it reduces susceptibility to the bends. This synthetic atmosphere is also used in medicine to relieve sufferers of respiratory difficulties because helium moves more easily than nitrogen through constricted respiratory passages. In surgery, beams of ionized helium from synchrocyclotron sources are proving useful in treating eye tumors, by stabilizing or even shrinking the tumors. Such beams are also used to shrink blood-vessel malformations in the brains of patients.