sucessive ion. energy and subshells

[lepton_index]

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Syllabus point: describe how trends in successive ionisation energies are used to predict the number of electrons in the outermost shell and the subshells occupied by these electrons

How can successive ionisation energies be used to predict the number of electrons in the subshells ???? I don't get it.
Please help,
Thanks

[tomorrows_angel]

neither do i, pm cameron0110 (sorry cam!!) if you're really desperate... he's really good at chem!

[cameron0110]

The idea of the ionisation energies in subshells is that electrons have a tendency to form lower spin subshells- where all the orbitals only have one electron all in the same spin, this means that the ionisation energy will not increase by a lot for the removal of that electron. The main indicator though is when the subshell is completely emptied because then the ionisation energy will increase dramatically because the next subshell is closer to the nucleus and the increased nuclear charge will mean the energy needed to remove the electron is dramatically increased.

Sorry that was a bit long-winded but try to decipher it. Btw the obvious point is that subshells are all at different energy levels when the atom has more than one electron.

[jang]

but successive ionisation energy doesnt hav obvious trends that relate to no of electrons in subshells. I know first ionisation energies of successive ELEMENTS reflect composition of electrons in subshells, but i dont think successive IE has nefin to do w/ it. tell us about the trends weber

[lepton_index]

To Cameron0110:
Are you sure you not confused between subshells and shells? What you said, I think, applies for shell.
The syllabus says: "... to predict the number of electrons in the outermost SHELL AND the SUBSHELLS occupied by these electrons".
Using the ionisation energy trend, we can predict the number of electrons in the outermost shell. Base on that, we can vaguely predict the subshells --- using Hund's rule.
I'm just wondering if there is any other, better method of predicting subshells.

[nit]

The way to predict the subshells occupied is rather dodgy, but there will certainly be a jump moving from removing, for example, a p-orbital electron to an s electron - the s electrons tend to penetrate closer to the nucleus than the p, d etc electrons, and so in moving from one orbital to the other the energy required will certainly increase more markedly than simply the increase associated with pulling off an electron from a greater positive charge on the ion...it is a dodgy way of determining the occupied subshell, but im pretty sure that that is what BOS wants

[cameron0110]

Originally Posted by lepton_index

To Cameron0110:
Are you sure you not confused between subshells and shells? What you said, I think, applies for shell.
The syllabus says: "... to predict the number of electrons in the outermost SHELL AND the SUBSHELLS occupied by these electrons".
Using the ionisation energy trend, we can predict the number of electrons in the outermost shell. Base on that, we can vaguely predict the subshells --- using Hund's rule.
I'm just wondering if there is any other, better method of predicting subshells.

Sorry if I wasn't clear about this, but I think nit has got it pretty much right. Basically the subshells on elements above Hydrogen i.e. everything that isn't hydrogen- exist at different energy levels due to repulsion from electrons and electron spin factors and so on. This basically means that because they are at different energy levels and the electron is removed from the outermost energy level then there will be a significant increase in ionisation energies in between different subshells and an even larger increase between different shells.