Topic 3 Extension

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You haven’t been asked to know this, but It widened my understanding of this topic, and I found a question that actually asked about it: Second ionisation energies.


Well, you do know that the second ionisation energy is the energy needed to remove a second electron from 1 mole of single charged ions. An example:


Na+ (g) à Na2+ (g) + e-


Well, the core of this extension thingy is this graph:

We can deduce two main things:-


1)     The second ionisation energy is ALWAYS higher than the first ionisation energy.


2)     The pattern it takes is similar to the first ionisation energy, but one step in front. This means that the rules that we used for the first ionisation energies will not apply to the same elements for the second ionisation energies.


Well, to clear things up, here’s an example:


-Why is the FIRST ionisation energy of Magnesium higher than Aluminium? and, why is the SECOND ionisation energy of Magnesium smaller than Aluminium?

Well, you should already know the answer for the first part. Look at the electronic configuration of Magnesium and Aluminium:


Mg       1s2 2s2 2p6 3s2


Al         1s2 2s2 2p6 3s2 3p1


It is easier to remove an electron from a partially full subshell (3p1) than a full subshell (3s2)


So, what about the second ionisation energy?


Mg+    1s2 2s2 2p6 3s1


Al+      1s2 2s2 2p6 3s2


It is easier to remove an electron from a partially full subshell (3s1) than a full subshell (3s2)


The question in the past papers asked about which of the first 20 elements has the highest second Ionisation energies. You should know now, from the chart above, that the answer is Lithium.


What confused me about this question was this: Doesn’t an overall increase in positive charge have an effect on ionisation energy? For this reason, I thought the answer was helium, because it only had 1 electron left, and it was closer to the nucleus than the electron of helium. Apparently, from the explanations above, the fullness of sub shells is more important in determining ionisation energy than net positive charge.


The above explanations can also be applied to all other ionisation energies.