Filling the 3s, 3p, and 4s Orbitals

The 3s and 3p orbitals are filled in going across the third period of the periodic table from sodium through argon. Atoms of these elements, like all atoms beyond neon, have their 10 innermost electrons in the neon electron configuration of ls 2s 2p. Therefore, these atoms have a neon core, which may be designated Ne  

With this notation, the electron configuration of element number 11, sodium, may be shown as NelSsi, which is an abbreviation for The former notation 

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