Online PES simulator
PES = PhotoElectron Spectroscopy
If you blast a sample with a lot of energy (light), you can pull the electrons off the nucleus.
If you know how much energy you put in, then the amount they come out with tells you how much it took to pull them off.
The height of a peak tells you how many electrons have that energy.
This basically gives you an electron configuration.
More Advanced Comparisons:
(These are crude approximations)
1. If you compare electrons in the same orbital from one element to another, the energy can change dramatically.
Mostly this just comes from having more or less protons 'holding onto' those electrons.
2. If you compare up and down a column, you see that the valence electrons in bigger atoms are easier to pull off (lower energy) than smaller atoms in the same group.
This is really about location and 'screening'. The valence electrons on the big atoms are far away from the nucleus and can't 'see' the full charge on the nucleus because of the other electrons in between.
Example PES spectrum for Nitrogen.
The easiest electrons to pull off are the three in the 2p
(note that the peak is about 1.5 times bigger)
Then the two electrons in the 2s
The two electrons in the 1s are the hardest
An Interesting Note:
If you look at the transition metals, you see that the 4s electrons tend to come off more easily (and are therefore removed first) compared to the 3d. The electrons come off in the sensible order, rather than the reverse of the order they went in.