Ionization energy

[pm1]

Lithium's 1st and 2nd ionization energies are 519kJ/mol and 7300 kJ/mol, respectively. Element X has a first ionization energy of 590 kJ/mol and a second ionization energy of 1150 kJ/mol. Element X is the most likely to be:

A. Oxygen
B. Sodium
C. Calcium
D. Xenon

Answer: C

I chose A. My reasoning was..
Sodium would have to have a bigger jump since the second ionization energy would be taking it out of a like noble gas state.
Calcium would have a lower second ionization energy since it would attain a noble gas like state by having its second electron removed.
Xenon would have an extremely large first ionization energy.

Then, O would just follow the normal pattern of the second ionization being higher than the first due to the already + charge from the first electron removal.

Wouldn't C have a lower second ionization energy?

Thank you!

---

Members don't see this ad.

 

[gettheleadout]

So for Element X we're looking at a relatively linear increase in IE from 1st to 2nd. Let's consider the choices:

A. Oxygen - First ionization would take O from 2p4 to 2p3, granting it increased electronic stability due to a half-filled p-subshell. We should expect a noticeably greater 2nd IE than a typical linear increase from the 1st IE, so this is unlikely to be correct.

B. Sodium - First ionization gives us noble gas configuration for the sodium ion, so we should expect a massively greater 2nd IE from than a typical linear increase from the 1st IE. This is definitely incorrect.

C. Calcium - First ionization puts us one away from noble gas configuration, so we know the electronic state acquired after second ionization gives us a very stable ion. We should then expect the 3rd IE to be very, very large. However, this is not relevant to the problem because we are only given the first two IE's. The key to this answer choice is knowing that even though the 2nd IE takes us from a less-stable ion (Ca+) to a very stable ion (Ca2+), the 2nd IE should still be greater than the 1st IE, because the effective nuclear charge of Ca+ is greater than Ca. It will still take more energy to remove the remaining valence electron than the first, even if the product is more than that formed when less energy was required. A linear increase should be expected here from 1st to 2nd IE. This is the best answer so far.

D. Xenon - First ionization of a noble gas is extremely unfavorable, so the 1st IE for Xenon should be much greater than that of Lithium. This is definitely incorrect.

Thus, C is the best answer.

 

[pm1]

So for Element X we're looking at a relatively linear increase in IE from 1st to 2nd. Let's consider the choices:

A. Oxygen - First ionization would take O from 2p4 to 2p3, granting it increased electronic stability due to a half-filled p-subshell. We should expect a noticeably greater 2nd IE than a typical linear increase from the 1st IE, so this is unlikely to be correct.

B. Sodium - First ionization gives us noble gas configuration for the sodium ion, so we should expect a massively greater 2nd IE from than a typical linear increase from the 1st IE. This is definitely incorrect.

C. Calcium - First ionization puts us one away from noble gas configuration, so we know the electronic state acquired after second ionization gives us a very stable ion. We should then expect the 3rd IE to be very, very large. However, this is not relevant to the problem because we are only given the first two IE's. The key to this answer choice is knowing that even though the 2nd IE takes us from a less-stable ion (Ca+) to a very stable ion (Ca2+), the 2nd IE should still be greater than the 1st IE, because the effective nuclear charge of Ca+ is greater than Ca. It will still take more energy to remove the remaining valence electron than the first, even if the product is more than that formed when less energy was required. A linear increase should be expected here from 1st to 2nd IE. This is the best answer so far.

D. Xenon - First ionization of a noble gas is extremely unfavorable, so the 1st IE for Xenon should be much greater than that of Lithium. This is definitely incorrect.

Thus, C is the best answer.

ahh that makes sense!! with the effective nuclear charge being larger, hence larger 2nd IE.

Very clever insight about the half filled p subshell!

Thank you! 👍