Question

Which element in the periodic table has the highest first ionisation energy?

Answer 1

Helium (He) has highest ionisation energy in periodic table.

Answer 2

The first ionization energy varies in a predictable way across the periodic table. The ionization energy decreases from top to bottom in groups, and increases from left to right across a period. Thus, helium has the largest first ionization energy, while francium has one of the lowest.

From this trend, Cesium is said to have the lowest ionization energy and Fluorine is said to have the highest ionization energy (with the exception of Helium and Neon). It is because of the shielding effect that the ionization energy decreases from top to bottom within a group.