Among silicon Si,calcium Ca,chlorine Cl,iron Fe and sodium Na which has largest second ionisation energy ?

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Explanation: 

Ionisation energies gets very large once we try to remove the core electrons,which are attracted more strongly to the nucleus than the valence electrons.So an atom with a very high second ionisation energy would be expected to have just 1 valence electron ; The second electron to be removed from such an atom would have to be a core electron .Among the choices,only sodium atos have a single valance electron.So, the second electron that we remove from sodium would be core electron and certaily will have the largest second ionisation energy among all .

Answer: 

Sodium will have the largest second ionisation energy since the second electron will come from the core shell.