Envision that you have nearly finished 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.

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Noble Gas Configuration

Sodium, element number 11, is the first element in the third period of the periodic table. Its electron configuration is (1s^2 2s^2 2p^6 3s^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 (left( Z=10 ight)). 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 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 (left< ceNe ight>) for the (1s^2 2s^2 2p^6) part of the configuration. Sodium"s noble gas configuration becomes (left< ceNe ight> 3s^1). Table (PageIndex1) shows the noble gas configurations of the third period elements.

Table (PageIndex1): Electron Configurations of Third-Period Elements Element NameSymbolAtomic NumberNoble Gas Electron Configuration
Sodium (ceNa) 11 (left< ceNe ight> 3s^1)
Magnesium (ceMg) 12 (left< ceNe ight> 3s^2)
Aluminum (ceAl) 13 (left< ceNe ight> 3s^2 3p^1)
Silicon (ceSi) 14 (left< ceNe ight> 3s^2 3p^2)
Phosphorus (ceP) 15 (left< ceNe ight> 3s^2 3p^3)
Sulfur (ceS) 16 (left< ceNe ight> 3s^2 3p^4)
Chlorine (ceCl) 17 (left< ceNe ight> 3s^2 3p^5)
Argon (ceAr) 18 (left< ceNe ight> 3s^2 3p^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 the use ofa different noble gas. Potassium has nineteen electrons, one more than the noble gas argon, so its configuration could be written as (left< ceAr ight> 4s^1). In a similar fashion, strontium has two more electrons than the noble gas krypton, which would allow us to write its electron configuration as (left< ceKr ight> 5s^2). All elements can be represented in this fashion.

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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.