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Question
Which element among the following has the highest ionisation energy: fluorine, oxygen, neon.

I know that all these elements belong to period 2 and ionisation energy increases from left to right across a period. But is there going to be an exception for neon? Or is it that Neon will also follow the normal rule?

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    $\begingroup$ I down voted because it does not show good research skill, as this answer can be found by simply searching on Google. $\endgroup$ – Abhigyan Chattopadhyay Mar 2 '18 at 2:35
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Compared with oxygen and fluorine, neon has to ionize an electron from the same subshell (2p) and there is more nuclear charge holding that subshell down. Neon thus requires the highest energy to ionize.

Now consider the process of accepting an electron. Oxygen and fluorine can do so in their 2p subshell but in neon that is full, the electron has to go into a higher subshell like 3s. That limits the electron affinity of neon.

So neon is hard to knock an electron out and yet does not release much energy accepting an electron. Add that up and you get ... an essentially inert element! Like ... neon.

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