# Between O- and F- which has higher electron affinity?

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O-- 2s2 2p5
F-- 2s2 2p6 (Fully filled hence more stable.)
Since F- is fully filled as compared to O- wouldn't it require more positive electron gain enthalpy to gain one more electron than O-?

However, the correct answer is given (A)

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Wrichik Basu
Gold Member
Since F- is fully filled as compared to O- wouldn't it require more positive electron gain enthalpy to gain one more electron than O-?
Don't think in that way.

$O^{-}$ requires one more electron to complete its stable configuration of [Ne]. On the other hand, $F^{-}$ already has the stable electronic configuration. So, it will not want to accept one more electron.

So, the electronic affinity of $O^{-}$ is greater than that of $F^{-}$.

When thinking about electronics affinity, you will have to consider which species will become more stable after the addition of one more electron. The stability before the addition of electronic should not be considered.

Don't think in that way.

$O^{-}$ requires one more electron to complete its stable configuration of [Ne]. On the other hand, $F^{-}$ already has the stable electronic configuration. So, it will not want to accept one more electron.

So, the electronic affinity of $O^{-}$ is greater than that of $F^{-}$.
But isn't that the exact same reason why putting an electron in F- absorb higher amount of energy than O- will? Am I correct?

Wrichik Basu
Gold Member
But isn't that the exact same reason why putting an electron in F- absorb higher amount of energy than O- will? Am I correct?
I've added a small explanation at the end:
When thinking about electronics affinity, you will have to consider which species will become more stable after the addition of one more electron. The stability before the addition of electronic should not be considered.
Does that help?

Don't think in that way.

When thinking about electronics affinity, you will have to consider which species will become more stable after the addition of one more electron. The stability before the addition of electronic should not be considered.
No one has ever told me that before. Can you also tell me why that is so?

Wrichik Basu
Gold Member
No one has ever told me that before. Can you also tell me why that is so?
You're also forgetting that electronics gain enthalpy is the amount of energy released on addition of one electron to a species. Released, not absorbed. $O^{-}$ will release energy, whereas $F^{-}$ will require energy to take in one more electron.

If any species becomes unstable after taking in one more electron, it will try not to do that. $F^{-}$ already has stable configuration. Why would it disturb itself unnecessarily? $O^{-}$ requires one more electron to be stable, so it will readily do that.

You're also forgetting that electronics gain enthalpy is the amount of energy released on addition of one electron to a species. Released, not absorbed. $O^{-}$ will release energy, whereas $F^{-}$ will require energy to take in one more electron.

If any species becomes unstable after taking in one more electron, it will try not to do that. $F^{-}$ already has stable configuration. Why would it disturb itself unnecessarily? $O^{-}$ requires one more electron to be stable, so it will readily do that.
But isn't the second EGE always positive, so for O- and F- both will be positive. Hence energy is absorbed.
Also, since F- is more stable, it would require more energy to take one more electron. So..

Regarding this:
If any species becomes unstable after taking in one more electron, it will try not to do that. $F^{-}$ already has stable configuration. Why would it disturb itself unnecessarily? $O^{-}$ requires one more electron to be stable, so it will readily do that.
Yes it will readily do that, but the energy (+ve hence absorbed) will be lesser. Isn't that the whole point?
Sorry if I'm bugging you, but I think there is something I still miss.
THank you for taking such great pains.

So higher positive EGE is actually lower negative EGE? Is that it?
THank you.

Wrichik Basu
Gold Member
My mistake

Yes, you're right. The second EGE of both Oxygen and Fluorine are positive. Both need more energy to take in one more electron. For oxygen, the size is small, and hence, another electron has to be pushed into the atom forcibly.

But fluorine has a higher second EGE than oxygen, making it more difficult (that is, more energy has to be supplied) to accommodate one more electron.