electron affinity

[tym]

Is electron affinity usually negative or positive? I've found it positive on the wikipedia page (http://en.wikipedia.org/wiki/Electron_affinity) and negative on some other sources (http://wiki.answers.com/Q/Why_is_io...ectron_affinity_is_sometimes_zero_or_negative).

If it is negative, is the periodic trend based on its absolute values? (trend: EA increasing from left to right and from bottom to top of the periodic table)

Thank you! Any suggestion will be greatly appreciated.

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[benjaminl1nus]

For me, it was helpful to think of electron affinity in this way.

Electron affinity describes this process - adding an electron to an atom, i.e. adding an electron to Fluorine: e + F ---> F-. can you think of what the reverse process may be? The reverse process is commonly known as ionization energy.

So electron affinity is related to how much energy it takes to add an electron to an atom. For the halogens, this process is so favorable that it results in a RELEASE of energy, aka a negative change in energy of the reaction; in other words, it takes a whole lot of energy to remove an electron from Fluorine atom that has already reached an octet.

Increasing electron affinity trend is another way of saying that it is getting easier to add an electron to an atom. This may be confusing because increasing ionization energy is another way of saying it's getting harder to remove an electron from an atom.

With regards to the negative or positive convention, the wikipedia page seems to be using the magnitude of the electron affinity, and this is useful for analyzing/describing trends.

Hope this helped!

 

[tym]

For me, it was helpful to think of electron affinity in this way.

Electron affinity describes this process - adding an electron to an atom, i.e. adding an electron to Fluorine: e + F ---> F-. can you think of what the reverse process may be? The reverse process is commonly known as ionization energy.

So electron affinity is related to how much energy it takes to add an electron to an atom. For the halogens, this process is so favorable that it results in a RELEASE of energy, aka a negative change in energy of the reaction; in other words, it takes a whole lot of energy to remove an electron from Fluorine atom that has already reached an octet.

Increasing electron affinity trend is another way of saying that it is getting easier to add an electron to an atom. This may be confusing because increasing ionization energy is another way of saying it's getting harder to remove an electron from an atom.

With regards to the negative or positive convention, the wikipedia page seems to be using the magnitude of the electron affinity, and this is useful for analyzing/describing trends.

Hope this helped!

Thanks benjaminl1nus!! That makes a lot of sense!
Other than halogens, can you think of other atoms that have negative electron affinities?
I found many electron affinity charts that have completely numbers. Do people have different ways of measuring them?

 

[FeinMS]

"To use electron affinities properly, it is essential to keep track of sign. For any reaction that releases energy, the change ΔE in total energy has a negative value and the reaction is called an exothermic process. Electron capture for almost all non-noble gas atoms involves the release of energy[4] and thus are exothermic. The positive values that are listed in tables of Eea are amounts or magnitudes. It is the word, released within the definition energy released that supplies the negative sign."

I'm pretty sure that all electron affinities are negative.

 

[tym]

General Chemistry Review by Princeton Review:
"If energy is released when the electron is added, the usual convention is to say that the electron affinity is negative; if energy is required in order to add the election, the election affinity is positive. Halogens have large negative electron affinity values." "The noble gases and alkaline earth metals have positive electron affinities."

So I guess there is both positive and negative values, and the trend is more negative as moving across a row.

 

[Blueprint MCAT Tutor]

Here's Wikipedia's take on it:

"Electron affinity can be defined in two equivalent ways. First, as the energy [that is] released by adding an electron to a gaseous atom. (The energy -or electron affinity- is a scalar[disambiguation needed] quantity and the direction of that energy -released- defines a reaction for which the change in energy ΔE is a negative quantity). The electron affinity is also defined in the case of electron capture as E(initial) – E(final) in order to maintain the positive value.[37] The reverse definition is that the electron affinity is the energy required to remove an electron from a gaseous anion (still a positive quantity, but in which the change in energy ΔE is also a positive quantity). Either convention can be used in practice, but must be consistent in according a scalar, i.e. positive number to the electron affinity. A negative sign appearing with a value for electron affinity indicates a change in direction, either that electron capture requires energy or removal releases energy."

 

[Blueprint MCAT Tutor]

I think for MCAT problem-solving purposes, they'll always give you e- aff as a positive number and that they'll only ask you cases where you're following the general periodic trends you learned in freshman chem.

So even though it gets more exothermic across the table (that is more and more negative), the numbers will get bigger, since by convention they're written as positives.