The noble gases are so stable, and react so little because, unusually for elements, the number of electrons in the outer part of the atom completely fill the shell. Not all of the noble gases are equally stable. For example, xenon can be made to combine with a few other elements, but helium combines with none. Hydrogen in water Water is the most common chemical compound on the surface of the Earth, covering over two-thirds of the Earth's surface. People are also about two-thirds water.
Water is a compound of hydrogen and oxygen.
Although we are so familiar with the way water behaves that we take it as normal, water, in fact, behaves differently from most other compounds. This is due to the strong attraction that each water molecule has for others. It is also the only common substance that swells as it freezes. All these unusual properties are due to the special way that the hydrogen atoms link water molecules together, the property called hydrogen bonding. The atoms that make up water molecules are bound together very much more firmly than are the molecules to each other.