What are unabbreviated and abbreviated electron configurations?

#He (Z=2):1s^2#

#Ne (Z=10):1s^(2)2s^(2)2p^6#

#Ar(Z=18):1s^(2)2s^(2)2p^(6)3s^(2)3p^6#

Now we know that on the basis of these fully occupied electronic configurations, the Noble Gases are supremely unreactive. Given this inertness, and the tedium of writing out the full electronic configuration, we could write out the electronic configuration of #Na#, #Z=11#, for instance as #[Ne]3s^1# rather than as #Na:1s^(2)2s^(2)2p^(6)3s^1.#

Of course it is only the valence electrons (for instance the #3s^1# electron of natrium) that are involved in reaction chemistry. The inert gas core are along for the ride.

Capisce?

An electron configuration shows the number and arrangement of the electrons in an atom based upon the period and s,p,d,f, orbital arrangement.

For instance Oxygen which has 8 elections would be

#O = 1s^2 2s^2 2p^4#

Magnesium has 12 electrons

#Mg = 1s^2 2s^2 2p^6 3s^2#

As we of further on the periodic table the lists would get longer and longer for instance Calcium has 20 electrons

#Ca = 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2#

Typically after the 18th element Argon we can use a short hand or abbreviated version of the system by using what is referred to as Noble Gas notation.

We use the Noble gas because we know the elements electron orbital are completely full to the point.

Using Calcium again

#Ca = **1s^2 2s^2 2p^6 3s^2 3p^6** 4s^2#

The bold section above is the exact same as Argon with 18 electrons

#Ar = **1s^2 2s^2 2p^6 3s^2 3p^6** #

We can therefore replace this section with the Noble Gas Notation [Ar] in the Calcium configuration.

#Ca = **[Ar]** 4s^2#

We always use the Noble Gas on the row above the element we are writing the configuration for.

One more example using Silver with 47 electrons

#Ag = 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3p^10 4p^6 5s^2 4d^9#

We use the Noble Gas above Silver which is Krypton [Kr]

#Ag = **[Kr]** 5s^2 4d^9#

Remember that the d block orbitals are one level lower then the period they are in on the periodic table.

Period 4 = 3d
Period 5 = 4d
etc...