Valence Electrons

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A chemical reaction involves either electron removal, electron addition, or electron sharing. Valence electrons are the electrons in the highest occupied principal energy level of an atom. In the second period elements, the two electrons in the 1s1s sublevel are called inner-shell electrons and are not involved directly in the element's reactivity or in the formation of compounds.

To tell how many electrons there are on a principal enegry level you use the formula 2n2. N stands for the energy level.