noble gases (Group 18), they have full electrons on the outer shell.
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The inert gases (noble gases) are the least reactive elements. They have completely filled p-sublevels, which is the lowest energy arrangement of electrons, and therefore, the inert gases don't react with any other elements in order to get to a lower energy level.
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The noble gases/Group VIII as they have full outer electron shells, therefore do not need to gain/lose an electron, meaning that they will have no reaction, and thus, are the least reactive.