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There are three different subatomic particles present in the atoms of each element: neutron, proton and electron. It is the electrons, and more specifically the valence electrons, that determine the reactivity of an element. Valence electrons are the electrons present in the outer electronic shells. These are the electrons that participate in bonding and are either shared or lost or gained. The elements with fully filled orbitals, such as the noble gases, are hardly reactive, due to their full shell of valence electrons. Elements lacking one or more electrons in the outer shells are very reactive, as they can attain fully filled orbitals by gaining an electronic or two. Halides are such examples. Similarly, elements with one or two electrons in the outer shell, such as alkaline earth metals, are also very reactive as they can give up these electrons to attain fully filled electronic configuration.
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