Question 14

[bfarahvash5364]

In the solution to this problem, it says that potassium has high electron affinity because it does not want to gain electrons. But a high electron affinity means that the atom wants to gain electrons rather than not wanting to gains electrons becasue accroding to the definition of electron affinity, it's the energy released when an atom gains electrons, so potassium would not release much energy when it gains electrons, which means it has a low electron affinity. So it has both low electron affinity and low ionization energy, and I don't understand how you can say which one is less than the other. could you please clarify this?

[mcat_premed3832]

You are referring to a technical issue that you would not be required to know on the real MCAT. In other words, you likely got this question correct even though you may not have been aware of the convention.

Nonetheless, EA is higher that IP since EA does not follow convention and is positive in value: http://en.wikipedia.org/wiki/Electron_affinity

[klee92181]

So on the MCAT, which definition of EA would they be referring to (amount of E required to take off an electron or amount of E released in adding an electron)?

Also the explanation said:

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.

Wouldn't having 2 valence electrons make it stable as well since the s subshell would be full?

Also I'm getting confused with like neutral potassium vs. K+. When you refer to neutral potassium are you talking about K0 (4s1 configuration) and K+ meaning noble gas configuration (3p6)?

Thanks

[admin]

I would stick with the definition given by Wikipedia but don't stress yourself over this, they will not try to trick you on this issue (no sign convention issues; this question is as tough as you will see and even if you did not know the convention, most will be guided to the right answer).

Regarding your question about filling the s subshell for K: by that logic, the second group of the periodic table should be very stable, yet, as you know, Mg and Ca love to lose 2 electrons (ie to be Mg++ and Ca++ respectively). Why? It's not about filling subshells, it's all about filling outer shells to increase stability like the noble gases.

You are correct in comparing the electronic configuration of K+ and neutral K.

The electron configuration for argon is: Ar 1s2 2s2 2p6 3s2 3p6

The electron configuration for potassium is: K 1s2 2s2 2p6 3s2 3p6 4s1

Potassium has an argon core plus 4s1 thus: in an abbreviated method for electron configurations a set of square brackets [ ] are placed around the chemical symbol of the noble gas.

The abbreviated electron configuration for K is: [Ar] 4s1

[jvuofm1709]

but potassium should still have a higher Ea than calcium right? It would gain more stability from having a full S shell than calcium would gain from adding one electron to its d shell.

[jellywing_2058]

The electron affinity is the potential energy change associated with the addition of an electron to a gaseous atom or ion in its ground state.

The ionization energy is the energy required to remove an electron from an isolated gaseous atom or ion in its ground state.

Potassium will not have a higher electron affinity (Ea) than calcium. In fact, potassium has one valence electron and the electron affinity energy is known to be -48 kJ/mol; however for calcium, it is +155 kJ/mol as it is in group 2 with two valence electrons.

Note also that both parameters are known to have periodic trends and the electron affinity roughly parallels the periodic trend of ionization energy.

[hua8986059]

i mean just based on trends, which is what we need to know for the MCAT electron affinity increases going to the right on the periodic table. So it makes sense that K actually have very low affinity therefore it requires (more energy to add on an electron than lets say Flourine.)

[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