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 right)). 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[ ce{Ne} right]) for the (1s^2 2s^2 2p^6) part of the configuration. Sodium’s noble gas configuration becomes (left[ ce{Ne} right] 3s^1). Table (PageIndex{1}) shows the noble gas configurations of the third period elements.
Table (PageIndex{1}): Electron Configurations of Third-Period Elements Element Name Symbol Atomic Number Noble Gas Electron Configuration Sodium (ce{Na}) 11 (left[ ce{Ne} right] 3s^1) Magnesium (ce{Mg}) 12 (left[ ce{Ne} right] 3s^2) Aluminum (ce{Al}) 13 (left[ ce{Ne} right] 3s^2 3p^1) Silicon (ce{Si}) 14 (left[ ce{Ne} right] 3s^2 3p^2) Phosphorus (ce{P}) 15 (left[ ce{Ne} right] 3s^2 3p^3) Sulfur (ce{S}) 16 (left[ ce{Ne} right] 3s^2 3p^4) Chlorine (ce{Cl}) 17 (left[ ce{Ne} right] 3s^2 3p^5) Argon (ce{Ar}) 18 (left[ ce{Ne} right] 3s^2 3p^6)
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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 of a different noble gas. Potassium has nineteen electrons, one more than the noble gas argon, so its configuration could be written as (left[ ce{Ar} right] 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[ ce{Kr} right] 5s^2). All elements can be represented in this fashion.
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