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Electrons and Sublevels  Electron Configurations and the Periodic Table  Writing Electron Configurations  Box and Arrow Configurations using Pauli Exclusion Principle and Hund's Rule   Quantum Numbers


Writing Electron Configurations



Sublevel Configuration- the distribution of the electrons in all the sublevels

Bohr Configuration-  the distribution of the electrons in the Principal Energy Level

Noble Gas Simplification- the distribution of the electrons in the sublevels, starting from the previous Noble Gas

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6

Auf bau Principle - you must add the lower energy sublevel before an electron can be placed in a higher one. So fill them in order.


Element# electronssublevel configuration
Magnesium  121s22s22p63s2
Aluminum              131s22s22p63s23p1
Silicon                    141s22s22p63s23p2
Phosphorus          151s22s22p63s23p3


Noble Gas Simplification

Now, this is becoming a lot of work writing down these ever-expanding electron configurations!  The more electrons we have in the atom, the longer the electron configuration.  We have a method we can use to simplify writing these configuration as we become more familiar with them.   If we look closely, we notice that the electron configuration for rubidium is the same as the previous element, argon, with a single 5s electron added on.  For the electron configuration of argon, let us simply write [Kr].  The simplified electron configuration for rubidium then becomes:



We can do this for any element, BUT, we must use only noble gases in the brackets.  I call this the noble gas simplification.  In this method of writing electron configurations, the last noble gas before we get to the element of interest is the noble gas we put into the brackets.  For instance, for the element aluminum we write

Sulfur [Ne]3s23p4
Chlorine [Ne]3s23p5

We may NOT use any element in the brackets, only noble gases. 

Valence Electrons-This notation for writing electron configurations helps us to highlight 2 different types of electrons in the atom.  Those electrons in the brackets are called core electrons.  These electrons do not participate in chemical reactions.   The electrons written after the noble gas in brackets are called valence electrons.   In many cases, “d” electrons will be present after the last noble gas, as in the element manganese :  [Ar]4s23d5.   We typically do not consider “d” electrons as valence electrons and therefore a more specific definition is needed: valence electrons are those electrons in the highest principal energy level.  These electrons are important because they are the ones that are gained, lost or shared in chemical reactions.  For the element aluminum, above, we see 2 electrons in the 3s orbital and 1 electron in the 3p orbital, so aluminum has a total of 3 valence electrons.  Using the same method, calcium has 2 valence electrons.   If we look at the electron configuration for manganese again:

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