Pb, Sn, Te, S, Cl
The ionization energy increases across a period but decreases down a group.
This means that the with the lowest ionization energies would be in the bottom left-hand corner of . The change in ionization energies is also bigger going down the periodic table (by change within a group) than going across the periodic table (by change within a period).
So let’s start from the bottom of the periodic table: ##Pb## is the element that is in the lowest period at 6 (and lowest group at 14) in the periodic table; it’s the smallest ionization energy.
The period above (5) has two of the elements: Sn and Te. Well, since ionization energy increases across a period, Sn will have a smaller ionization energy than Te. ##Pb, Sn, Te##
Now, let’s go to the third period, where ##S## and ##Cl## are. Since ##S## is before ##Cl,## ##S## has a lower ionization energy than ##Cl##. ##Pb, Sn, Te, S, Cl##
