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I know that neon is used in advert signs due to its inertness. However, I am not entirely sure how the inertness is exploited. I think it is because Ne being inert means that after electricity frees an electron we can count on it returning to the ion left behind to emit a photon, whereas for more active elements, the gas may just exist as a plasma?

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    $\begingroup$ Ne is used, 1. because it caused the red glow inside the tube. 2. because even when it exist as plasma, it doesn't react with the filament inside the tube or the glass walls. $\endgroup$ Dec 15 '11 at 17:00
  • $\begingroup$ There's a similar reason why incandescent bulbs have a vacuum inside; at high heat and lots of electrons running amuck, most other gases tend to react with metals (oxides, nitrides, etc.), which tend to be less durable than the metals... $\endgroup$
    – user172
    Dec 15 '11 at 17:13
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    $\begingroup$ @VineetMenon and j-m: I know both of your answers are short, but you should really post them as answers to let the rest of us vote on them. Comments should not have answers but "requests for clarification". $\endgroup$
    – Arnoques
    Dec 16 '11 at 1:55
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Ne is used,

  1. Because it caused the red glow inside the tube, infact you can get a whole array of colors using different noble gases. eg. Ne => Red, Xe => Whitish Blue, Ar => Blue etc. Check Wikipedia for more.
  2. Because even when it exist as plasma, it doesn't react with the filament inside the tube or the glass walls, this helps in the longer life of lamp, and as pointed out by @J.M. this technique finds its use in incandescent lamp where Argon is generally used.
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