electron affinity

[fairylight]

Why are all the values of electron affinity positive except for the group 2 elements and N?

[rthmjohn]

You also forgot to mention the noble gases. The reason for this is because the process of removing electrons from the specified elements requires energy. The values are in Kj/mol of the substance. A negative value means that the removal process gives off energy.

[insane_alien]

electron affinity is the addition of electrons.

[RyanJ]

You also forgot to mention the noble gases. The reason for this is because the process of removing electrons from the specified elements requires energy. The values are in Kj/mol of the substance. A negative value means that the removal process gives off energy.

 

As insane_alien said its the reversal of that.

 

Adding an electron normally gives off more energy than it uses (Exothermic) but for the others it takes in more than it gives out (Endothermic).

 

In chemistry' date=' electron affinity is the amount of energy absorbed when an electron is added to a neutral isolated gaseous atom to form a gaseous ion with a -1 charge. It has a negative value if energy is released.

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http://en.wikipedia.org/wiki/Electron_affinity

 

Cheers,

 

Ryan Jones

[rthmjohn]

I'm sorry, I can't believe I posted that (trust me, I DO know what electron affinity is). Yes, electron affinity is the ADDITION of electrons. My bad, but the same holds true for the positive and negative values. Positive values mean endothermic and negative mean exothermic. Be careful how you read certain tables, though... Some tables erroneously list affinity values as positive when they should actually be negative and vice versa. ie. I'm sure Nitrogen is actually positive since it requires more energy to add an electron.

[RyanJ]

Nitrogen is actually positive since it requires more energy to add an electron.

 

Your right even though I can't seem too find much information on why this occurs :S

 

Cheers,

 

Ryan Jones

[silkworm]

Your right even though I can't seem too find much information on why this occurs :S

 

It all has to do with sublevels and why there are hiccups across the periodic table in general. Nitrogen has an atomic number of 7' date=' so it has 7 electrons, 2 of which are used in the 1s valence shell. There are 5 valence electrons, 2 of which are used in the 2s sublevel, and 1 each in the 2px, 2py, and 2pz sublevels (Aufbau Principle). Adding an electron at this point is pairing an unpaired electron in a p-orbital and for that to happen energy is required to overcome the repulsion.

 

Why are all the values of electron affinity positive except for the group 2 elements and N?

 

It's the same principle in group 2 as it is for Nitrogen, only here the addition is to its ns orbital (n being it's highest level achieved).

[rthmjohn]

Wouldn't it be more appropriate to ask why the electron affinities of group 2 elements and nitrogen are positive? Like I said, the process is endothermic, so wouldn't a positive value make more sense in describing the process?

[silkworm]

Generally, negative in chemical calculations means exothermic and spontaneous.

 

CORRECTION: On my previous post on this thread I flubbed and said "2 of which are used in the 1s valence shell." The 1s shell is not the valence shell, the 5 electrons remaining are in the valence shell. Since there are 5 the 2s sublevel is full and the 2px 2py and 2pz have one each and energy must be added at this point to overcome repulsion. I'm sorry if my use of the word valence for 1s cause confusion.