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Interpreting graphs of successive ionisation energies

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The graph below shows the successive ionisation energies of the element chlorine (atomic number 17).

• The first ionisation energy is labelled with an arrow.
• The 7 electrons from the outer shell of the chlorine atom (shown in blue) are the first to be removed.
• The 8 electrons from the next shell of the chlorine atom (shown in red) are the next to be removed.
• Finally, the two electrons from the inner shell of the chlorine atom (shown in green) are removed.

Note that in an A level exam you will not necessarily be shown all the successive ionisation energies of an atom, you may only be shown the first 10 or so, but this should still be enough information to deduce how many electrons the atom has in its outer shell and therefore to which group in the Periodic Table it belongs.