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During gradual filling of electrons in successive shells, the elements of the periodic table are reached. Starting from Helium, the first of the noble gases, electronic configuration of all elements can be expressed indicating the inner core as that of a noble gas. For example Li(3) -> [1s˄2] 2s˄1 or, [He]2s˄1; Be(4) -> [1s˄2] 2s˄2 or, [He]2s˄2; B(5) -> [1s˄2] 2s˄22p˄1 or, [He]2s˄22p˄1; C(6) -> [1s˄2] 2s˄22p˄2 or, [He]2s˄22p˄2; N(7) -> [1s˄2] 2s˄22p˄3 or, [He]2s˄22p˄3; O(8) -> [1s˄2] 2s˄22p˄4 or, [He]2s˄22p˄4; F(9) -> [1s˄2] 2s˄22p˄5 or, [He]2s˄22p˄5; Ne(10) -> [1s˄2] 2s˄22p˄6 or, [Ne]; Na(11) -> [1s˄2] 2s˄22p˄63s˄1 or, [Ne]3s˄1; Mg(12) -> [1s˄2] 2s˄22p˄63s˄2 or, [Ne]3s˄2 and so on. The symbols for noble gases and their atomic numbers are: He(2), Ne(10), Ar(18), Kr(36), Xe(54), and Rn(86).
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