Question 28

[yfangl097543]

What is the definition of electron affinity? Is it the energy release when an electron is acquired or the energy required to add an electron?

[admin]

Electron affinity is the energy required to remove an electron from a singly charged negative ion, i.e., the change in E for:

X- --> X + e-

The other definition is that electron affinity is the energy released (Einitial - Efinal) when an electron is attached to a neutral atom or molecule. The sign convention for electron aff. is the opposite to most thermodynamic quantities: a positive electron aff. indicates that energy is released when going from atom to anion.

[yfangl097543]

So in the question:

Why does potassium possess a higher first electron affinity than first ionization energy?

Is it saying that more energy is released when adding an electron to potassium than taking one off? This doesn't make sense...

[admin]

I think that you would agree that according to the periodic table, K has a very low IE.

So the question is: is it true that its EA is high?

Let's check. We can use either definition of EA, but to be most precise, let's use the second definition.

[yfangl097543]

So wouldn't the electron affinity be negative then, since you would have to INPUT energy, which means no energy is released when K goes to K-?

[admin]

According to the reference below, EA for the reaction in question is positive:

K + e- --> K-

[anon3543]

I think the REVERSE of "a positive electron aff. indicates that energy is released when going from atom to anion." is true, then everything makes sense... I understand that the positive/negative stuff is only a convention really, but according to everything else you (admin) said, it would be much better to assign release of energy as negative electron affinity. Anyways, that's just my thoughts.

[admin]

Bump.

[mohsinalam5969]

DEFINITION :
The term ionization energy of an atom or molecule means the energy needed to remove electrons from an atom
(Ionization energy is the enery it takes to lose an electron) In K, the electrons are relatively near the positive nucleus..

DEFINITION :
The Electron affinity of a molecule or atom is the energy change when an electron is added to the neutral atom to form a negative ion. This property can only be measured in an atom in gaseous state.
X + e− → X- ...
hey represent the energy involved in the same process, just reversed.

K+ + e- --> K for the electron affinity of potassium cation
K --> K+ + e- for the ionization energy of potassium metal
My thought is that the energy is same but with opposite signs.
I believe technically you could invoke the law of microscopic reversibility or the quantization of electronic states... but that's probably more complicated than necessary.

But if we have to prove that tha first electron affinity is greater than the ionization energy then there is no other reson except this as given below

Since potassium only has one electron in its outer shell, it is easily removed (i.e. low first ionization potential/energy), thus leaving potassium with the more stable noble gas-like configuration (i.e. Ar). On the other hand, adding an electron to neutral potassium would require much energy (i.e. high first electron affinity) which would leave potassium with two valence electrons which would not be stable.
THANKS