The ionizatin energy is the is the energy required to remove one electron from from a gaseous atom.
There is a clear trend for the first ionization energies (the ionization energy to remove the first electron) of the atoms.
Inside a group (a column of the periodic table) the ionization energy increases downwards .
Then, the element that has the greatest atomic number in the group has the highest ionization energy (it is more difficult to form its ion).
As per this trend, in the group 4A lead, Pb (atomic number 82) is the natural element with the highest ionization energy. But there is one artificial element in this group, whose atomic number is 114. It is called Flerovium and as per the rule it shall have a higher ionization energy.
