Arrange the elements in order of increasing ionization energy: Te, Pb, Cl, S, Sn.
Ionization energy is the energy required to remove the most loosely held electron from an isolated gaseous atom
Variation in ionization energy down a group
Ionization energy decreases down a group. This is because the atomic size increases down a group. Hence the effective nuclear pull experienced by the outermost electron will be lesser and it will be easy to remove it.
Variation in ionization energy across a period
As we move from left to right in a period, ionization energy increases. This is because size of the atoms decreases as we move towards the right .Hence the effective nuclear pull experienced by the outermost electron increase and it will be difficult to remove it.