Explain the reactivity of the noble gases in terms of valence electrons.

Valence electrons are the electrons in the outer shell of an atom and are the ones that participate in chemical bonding. Noble gases are elements, characterized by fully filled outer shells. They form group 18 of the periodic table of elements. Noble gases include helium, neon, argon, krypton, xenon and radon. These elements have the maximum number of valence electrons the outer shell can hold. For example helium has an atomic number of 2 and electronic configuration of 1s2. There are no free electrons and there is no need to gain, lose or share electrons, as the outer shell is fully filled. Similarly, neon has an electronic configuration of 1s2, 2s2, 2p6. Since the outer orbitals are fully filled, noble gases do not react and are mostly chemically inert. In comparison, metals and non-metals react and participate in chemical reactions to completely fill their outer shells. 

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