Page 43 - Curriculum Visions Dynamic Book

half-life: the time it takes for the radiation coming from a sample of a radioactive element to decrease by half.
inert: nonreactive. There are few substances that hardly react. They include all the rare gases (helium, neon, argon, krypton, xenon and radon) and also diamonds and quartz.
isotope: atoms that have the same number of protons in their nucleus, but which have different masses; for example, carbon-12 and carbon-14.
radioactive: a material that emits radiation or particles from the nucleus of its atoms.
Krypton
Krypton, symbol Kr, whose name comes from the Greek kryptos, meaning “hidden”, is the fourth of the noble gases in the periodic
table. It occurs as only one part in every million parts of air by volume.
As with other rare gases, for industrial purposes krypton is extracted by cooling
air until the gas turns to liquid. In the case of krypton this is -152°C. However, it is very difficult to isolate krypton this
way, so the gas is very expensive. This factor, more than any other, limits the number of uses to which krypton is
put. It is used in both discharge tubes and incandescent bulbs.
Krypton also has the unusual distinction of being the element whose
light is used to define the metre. Since 1960, scientists have defined the metre to be the distance of 1,650,763.73 wavelengths
of the orange–red line of krypton, atomic weight 86.
Radon
Radon, symbol Rn, is named after radium. It is the sixth member of the noble gases in the Periodic Table. Radon is very different from the other rare gases insofar as it is radioactive, produced by the metal element radium. Radon gas is common worldwide, although it is more concentrated in areas where there are igneous rocks such as granites. The surface soils of the world contain about one gram of radium-emitting radon in every three square kilometres.
The half-life of radon is just under four days. The main form of radiation is alpha particles, the least harmful and least penetrating form of radiation.
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