Second Ionization energy

[Pantaloons]

Here's a question I came across:

"The second ionization energy is defined as the minimum amount of energy needed to remove an electrom from a gaseous +1 cation in its ground state. Which of the following must be true of any element?

 

A) Second Ionization energies are always higher than first ionization energies

Second Ionization energies may be higher or lower that first ionization energies.

Clearly, the obvious choice is A, since we all know the periodic trend.

-->It takes more energy to remove an electron from a cation than from a neutral atom. Further, the second electron must have had lower energy than the first electron, so the second electron must require even more energy for removal than the first one.

 

 

But, then I started second guessing myself: What if you had an atom like Magnesium?

 

Doesn't magnesium want to have an octet, so wouldn't removing the second electron be a very favourable process, and thus require little energy?

Or...are favourability of an octet and Ionization energy two completely different issues?

 

Thanks!

[The Law]

You are right about "octets" stabilizing an atom and making its ionization energy higher. In magnesium's case, it isn't really an octet since magnesium's electrons are only occupying the s-orbital (the same pattern still applies with a full orbital, however). If you think about it though, I am not 100% sure, but I would say the positive charge found on the atom far outweighs the stable orbital state. Electrons are primarily affected by their attraction to positive charges... the other quantum states do affect things, but not as much as their love for their other half (the proton).

 

Edit: The need for a stable orbital does affect things though... if you consider for example the first ionization energy of magnesium and aluminum. Magnesium has a full s-orbital, while aluminum has a full S-orbital and a half fulled p-orbital. It is easier to pull off the first electron off aluminum than it is magnesium because it isn't disrupting a full orbital. So in summary... for a specific atom, 2nd ionization energy is always larger than first... but recognize when making comparisons that removing electrons from stable orbitals requires more energy than not doing so.