Chlorine vs. Fluorine Electron Affinity

[Lunasly]

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According to a few sources, chlorine has a higher electron affinity than fluorine. Why does chlorine have a higher tendency to gain an electron than fluorine? If such a question on the MCAT were to arise (i.e., electron affinity comparison between fluorine vs. chlorine) should I pick chlorine?

 

[Teleologist]

What are the relative electron densities of fluorine vs chlorine?

 

[Lunasly]

Ah, ok. So, fluorine is an exception because it has higher electron density given that is smaller. Therefore, it is more unfavourable relative to chlorine to add an electron since there would be more electron repulsion. Right?

Thanks for the help. 🙂

 

[getfat]

I thought Cl > F was just an exception to the general rule for electron affinity because
Cl is larger than fluorine and is able to handle an extra electron better than fluorine.

 

[Teleologist]

I thought Cl > F was just an exception to the general rule for electron affinity because
Cl is larger than fluorine and is able to handle an extra electron better than fluorine.

Br is larger than Cl and F; how come Br's electron affinity is less than either?

 

[getfat]

Br is larger than Cl and F; how come Br's electron affinity is less than either?

From my understanding, it was in most cases electron affinity is UP and to the RIGHT.
But when comparing Cl to F, there is an exception; Cl is more electronegative because it is larger than F and can handle the charge.

 

[Teleologist]

But when comparing Cl to F, there is an exception; Cl is more electronegative because it is larger than F and can handle the charge.

Cl ain't more EN than F.

 

[getfat]

Sorry ment to say Electron affinty i higher in Cl than F, thanks

 

[Teleologist]

Sorry ment to say Electron affinty i higher in Cl than F, thanks

You're also conflating electron affinity with electronegativity. The two have distinct meanings.

 

[csx]

so is there a definitive explanation/answer? you guys are all over the place

 

[Teleologist]

so is there a definitive explanation/answer? you guys are all over the place

Yes, and it involves:

1) Learning what electron affinity and electronegativity are
2) Learning what the difference between the two terms is

 

[Lunasly]

Yes, electron affinity generally increases as you move to the right and up the periodic table, however, the exception is with fluorine vs. chlorine. Chlorine has greater electron affinity because its electron cloud is not as dense as fluorines. Therefore, it has a greater tendency relative to fluorine to accept an electron (more favorable).

Teleologist is right though. Electronegativity is not the same as electron affinity. Electron affinity is the energy associated with gaining an electron. Electronegativity is defined as the tendency to hold closer shared electrons with another atom within a covalent bond. They are different principles.

 

[csx]

Yes, electron affinity generally increases as you move to the right and up the periodic table, however, the exception is with fluorine vs. chlorine. Chlorine has greater electron affinity because its electron cloud is not as dense as fluorines. Therefore, it has a greater tendency relative to fluorine to accept an electron (more favorable).

Teleologist is right though. Electronegativity is not the same as electron affinity. Electron affinity is the energy associated with gaining an electron. Electronegativity is defined as the tendency to hold closer shared electrons with another atom within a covalent bond. They are different principles.

I see where you are going, but if the electron cloud is less dense in Cl... wouldn't that just mean the outermost electrons feel less of an attraction from the protons in the nucleus due to shielding which in turn would cause repulsion leading to a reduction in its willingness to accept an electron?

still trying to wrap my head around this and I'm so close hahaha

 

[Lunasly]

I might have some of this wrong so someone might have to correct me, but this is how I see it...

What's important to realize is that Chlorine and Fluorine both want to accept an electron to achieve an octet electron configuration. Given that fluorine has its electrons (both core and valence) held closer towards the nucleus, it is denser. According to Coloumbs law, the closer you bring two charges towards one another, the greater the electrostatic force. Since the charges in this case (electrons) are both negative, the electrostatic force of repulsion increases. Therefore, to add another valence electron to an atom that is denser (has its electrons held tighter) will become more difficult as the electrostatic repulsion increases. Chlorine has a similar issue, but its electrons aren't held as tightly, so it is more favorable for it to accept an electron. Keep in mind that the tendency of either fluorine or chlorine to accept an electron is still very high. Both will do it since achieving an octet is thermodynamically favorable. It just turns out that its more favorable for chlorine to do it relative to fluorine.

 

[Teleologist]

According to Coloumbs law

Thank GOD for bringing "Coloumbs [sic]" law into this discussion. If we'd all go back to first grade, then we'd immediately realize that opposites attract, and likes repel. Problem solved, end of story.

 

[Chrisz]

Lol, I think I just come up an analogy to visualize this situation. No offense intended, it is purely hypothetical. Four college students share a small apt to save some money, they only have one bathroom. They have to fight to use the bathroom every morning. One morning, one student sits on the toilet, while the second one brushes tooth and the third guy takes a shower. The forth one wanna come in and wash his face. The bathroom is already so crowded, so the first three students try to push him out before he comes in.

 

[csx]

I might have some of this wrong so someone might have to correct me, but this is how I see it...

What's important to realize is that Chlorine and Fluorine both want to accept an electron to achieve an octet electron configuration. Given that fluorine has its electrons (both core and valence) held closer towards the nucleus, it is denser. According to Coloumbs law, the closer you bring two charges towards one another, the greater the electrostatic force. Since the charges in this case (electrons) are both negative, the electrostatic force of repulsion increases. Therefore, to add another valence electron to an atom that is denser (has its electrons held tighter) will become more difficult as the electrostatic repulsion increases. Chlorine has a similar issue, but its electrons aren't held as tightly, so it is more favorable for it to accept an electron. Keep in mind that the tendency of either fluorine or chlorine to accept an electron is still very high. Both will do it since achieving an octet is thermodynamically favorable. It just turns out that its more favorable for chlorine to do it relative to fluorine.

Is this Cl>F electron Aff and exception to the trend? which according to TPR the EA tends to increase as you go up and to the right. No real reason explained for Cl>Fl in my book nor is it mentioned.

 

[Teleologist]

Is this Cl>F electron Aff and exception to the trend? which according to TPR the EA tends to increase as you go up and to the right. No real reason explained for Cl>Fl in my book nor is it mentioned.

Yes it's an exception.