Noble or inert gases have the most stable electronic configuration. All elements try to achieve inert gas configuration in their outermost shell (or valence shell) during combination with other elements to form compounds. During gradual filling of electrons in successive shells, successive 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 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).
Again, the elctronic configuration of iron is Fe(26) -> [1s˄22s˄22p˄63s˄23p˄6]3d˄64s˄2 or, [Ar]3d˄64s˄2. Note the filling of 4s (n+l=4+0=4) orbitals before 3d (n+l=3+2=5) orbitals.