Page 5 - Curriculum Visions Dynamic Book

The noble gases
Helium (symbol He) is another element with a simple structure. It is the second most abundant element in the Universe,
and yet its presence was not suspected until relatively
recently. It is the only element ever to have been
identified in space before it was found on Earth. Scientists noticed a mysterious element they could not identify in the light shining from stars. Eventually this was identified as helium.
Helium, like hydrogen, is lighter than air. But it is an inert gas, which means it rarely reacts and cannot burn. Being inert, it is not found in any compounds on Earth, which is why it took so long to identify. But being an inert gas also means it can be used in places where hydrogen would be dangerous. Thus, helium, rather than hydrogen, fills the floating balloons seen at funfairs.
Helium is one of a group of inert, or
noble, gases, whose physical and chemical properties are closely related. The other
noble gases, which together make about 1%
of the Earth’s atmosphere, are neon, argon, krypton, xenon and radon. All other elements
react to form stable substances. The noble
gases are unreactive because they each have just enough electrons to be perfectly stable on their own, without needing to react with other elements.
 The immense heat of the Sun is produced by burning hydrogen. 5