Open shell
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In the context of atomic orbitals, an open shell is a valence shell which is not completely filled with electrons or which has not given all of its valence electrons through chemical bonds with other atoms or molecules during a chemical reaction. The valence shell of most common elements must contain eight electrons to be completely filled (at least for Periods 2 and 3). This outer shell is filled (or emptied by giving up valence electrons) when establishing chemical bonds. Notice that the outer shell for hydrogen which is in Period 1 may contain only two electrons as opposed to the previously mentioned eight electrons because it is the first shell (s orbital) and the maximum number of electrons that can be contained in this shell is 2 electrons. H2 has therefore a stable covalent bond. The helium atom, also, has only two electrons in its outer shell and has the configuration of a noble gas. As well as all other noble gases it is usually not reacting with other elements and thus very stable.
This is a very important consideration. Having an open shell means that one or more electrons in the outer shell of the element is (are) available to react with an external atom or molecule to establich a chemical bond and is therefore unstable. When a reaction fills an open shell or removes all of its valence electrons, it releases energy and the resulting product is at a lower energy level and more stable.
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