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All elements want a full outer shell, to make them very stable.
Compounds are made by elements sharing, donating or receiving electrons.
Elements will donate or accept electrons in order to have an octet (eight) in their outer shell (with the exception of hydrogen and helium which only need two to fill the shell). For example, sodium has 11 electrons, 2 in the first shell, 8 in the second shell and 1 in the third shell. Sodium is willing to donate that one electron in it's third shell, because then the third shell disappears, and it will have 8 in it's second (which is now the outer) shell. Sodium now has a positive one charge because it is minus an electron (minus a negative is positive). Chlorine is the opposite, it has seven electrons in its outer shell, so it needs one to complete the octet. It gladly accepts the electron that sodium donates, becoming negatively charged in the exchange. Now sodium has a positive charge, chlorine has a negative charge and we know that opposites attract.
Neon has eight electrons in its outer shell, it doesn't want to donate any, it doesn't want to accept any therefore it is unreactive. All the noble gases have full outer shells, so they are all unreactive. Sometimes the noble gases are called "inert" because of their unreactiveness.
Brown and LeMay, CHEMISTRY THE CENTRAL SCIENCE, 9th edition, Prentice Hall.
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