Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
Rina plots the first ionisation enthalpy against Z for the first 20 elements. How many sharp peaks does she see, at which elements, and which elements make the deep troughs just after them?
First ionisation enthalpy peaks at the noble gases and falls to its lowest at the alkali metals that follow. Across a period the atomic radius falls, because the nuclear charge grows while the shell stays the same; down a group it rises, because a shell is added. diff() gives each value minus the one before.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Peaks. The noble gases have full shells: helium (Z ), neon (Z ) and argon (Z ). Three peaks.
Step 2. Troughs. Each is followed by an alkali metal with one electron in a new shell: lithium, sodium and potassium.
Answer: peaks, at He, Ne and Ar; troughs at Li, Na and K.
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