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# Noble Gas Configuration

## Shortening electron configurations using symbols.

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Noble Gas Configuration

How does it feel to be full after a meal?

You had a great meal, but cannot put another bite in your mouth because there is no place for it to go. The noble gases have the same problem – there is no room for any more electrons in their outer shells. They are completely full and cannot handle any more.

### Noble Gas Configurations

Sodium, element number eleven, is the first element in the third period of the periodic table. Its electron configuration is 1 s 2 2 s 2 2 p 6 3 s 1 . The first ten electrons of the sodium atom are the inner-shell electrons and the configuration of just those ten electrons is exactly the same as the configuration of the element neon (Z = 10). This provides the basis for a shorthand notation for electron configurations called the noble gas configuration. The elements that are found in the last column of the periodic table are an important group of elements that are called the noble gases. They are helium, neon, argon, krypton, xenon, and radon. A noble gas configuration of an atom consists of the elemental symbol of the last noble gas prior to that atom, followed by the configuration of the remaining electrons. So for sodium, we make the substitution of [Ne] for the 1 s 2 2 s 2 2 p 6 part of the configuration. Sodium’s noble gas configuration becomes [Ne]3 s 1 . Table below shows the noble gas configurations of the third period elements.

 Element Name Symbol Atomic Number Noble Gas Electron Configuration Sodium Na 11 [Ne]3 s 1 Magnesium Mg 12 [Ne]3 s 2 Aluminum Al 13 [Ne]3 s 2 3 p 1 Silicon Si 14 [Ne]3 s 2 3 p 2 Phosphorus P 15 [Ne]3 s 2 3 p 3 Sulfur S 16 [Ne]3 s 2 3 p 4 Chlorine Cl 17 [Ne]3 s 2 3 p 5 Argon Ar 18 [Ne]3 s 2 3 p 6

Again, the number of valence electrons increases from one to eight across the third period.

The fourth and subsequent periods follow the same pattern except for using a different noble gas. Potassium has nineteen electrons, one more than the noble gas argon, so its configuration could be written as [Ar]4 s 1 . In a similar fashion, strontium has two more electrons than the noble gas krypton, which would allow us to write its electrons distribution as [Kr]5 s 2 . All the elements can be represented in this fashion.

#### Summary

• The noble gas configuration system allows some shortening of the total electron configuration by using the symbol for the noble gas of the previous period as part of the pattern of electrons.

#### Practice

Use the link below to practice writing noble gas configurations:

#### Review

1. What is the element represented by [Ne]3 s 2 3 p 2 ?
2. What element has this electron configuration [Ar]3 d 7 4 s 2 ?
3. What noble gas would be part of the electron configuration notation for Mn?
4. How would you write the electron configuration for Ba?