Respuesta :

SoapyMcTavish
It's the Helium. 
The Electron configuration of the Oxygen is [tex]1s^2 2s^2 2p^4[/tex]
And the Helium is represented by the  [tex]1s^2[/tex] 
So, the [tex]1s^2[/tex] of the oxygen can be replaced by Helium

Hope this Helps :) 

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