A noble gas does not readily combine into molecules. The colorless, odorless noble gases are helium, neon, argon, krypton, xenon and radon. They are gases because their molecules move freely at normal temperatures, not flowing like liquids or holding shape like solids. They are called noble because they were once believed to hold themselves apart from other elements, in noble fashion.
Some noble gases do form molecules, though they are much less reactive than other elements. The reason noble gases are so inert has to do with the composition of their outer electron shells.
Electron shells and molecules
Electrons surround the nucleus of an atom in layers of shells, which can be thought of as resembling the layers of an onion. Some people visualize electrons as tiny satellites that orbit the nucleus the way planets orbit the sun. However, electrons don't really make that kind of circular orbit. They make something more like a cloud or a shell.
The smallest possible outer shell can hold up to two electrons. When that shell is full, up to eight electrons will fit in the next outer shell. Atoms are more chemically stable when their outer shells are full.
Electrons tend to connect to other atoms in a molecule, which promotes stability. Some of the atoms in a molecule give up extra electrons until they peel down to a full outer shell. Other atoms in the molecule accept those electrons until they fill their outer shell.
Noble gases have outer shells that are already complete. Therefore they are usually found as single atoms, not combined into molecules.
Hydrogen and helium
The innermost shell of an atom can hold two electrons. Hydrogen, an explosive element, only has one electron in its shell, so hydrogen atoms usually combine with other atoms that have an extra electron to share with it. For example, when two hydrogen atoms combine with an atom of oxygen, the oxygen gives one electron to each hydrogen atom. As a result, the oxygen does not have too many electrons in its outer shell, and the hydrogen does not have too few. The result is a chemically stable molecule of water.
In contrast, an inert atom of helium has two electrons in its outer shell. It does not need to react with other atoms. Therefore, helium is usually monatomic, that is, it's found in single atoms, uncombined.
Helium is the lightest of the noble elements. It stands in the top right hand corner of the periodic table of the elements. The elements directly beneath it are the other members of group 18, the noble gases.
Other noble gases
Neon and argon are used to produce light. Argon fills fluorescent tubes, and neon makes brilliant red-orange light when electricity passes through it. Krypton gives off dazzling yellow-green light.
Krypton and xenon actually form molecules, though rarely. These heavier noble gases can react because their outer shells are relatively far from the nucleus and partly shielded from its binding power by the electrons in between.
In smaller atoms like helium, argon and neon, the powerful nucleus holds the outer shell's electrons in place. In larger atoms, they can occasionally break away and react with other atoms.
Radon makes compounds, but deadly radioactive radon is hard to study. It decays quickly, so less is known about its compounds.
Beneath radon in the periodic table, ununoctium appears to belong to the noble gas group, but it may not. Only three or four atoms of it have been created, so knowledge about it is largely theoretical. It may be solid at standard temperatures and may be more reactive than radon.
The noble gases are not reactive compared to other elements, because their outer electron shells are filled. Industry has found many uses for noble gases, and science continues to investigate their fascinating properties.