Question

which element has highest ionization energy? WHY?

Answer 1

helium has a highest ionization energy
Noble Gases have a very stableelectron configuration, therefore, they have the highest amount of ionization energy within their periods. It decreases down a family becauseelectrons farther from the nucleus are easier to remove. Largest to Smallest:Helium (highest ionization level in the periodic table)

Answer 2

Helium has the highest ionisation energy of 24.5874 eV.

Ionisation energy is the energy which is required to remove the electron from its outermost orbit of an isolated gaesous atom. The isolated gaseous atom which has high stability will require more I.E becuase in stable atoms force of attraction between the nucleus and electrons is high.

Since the noble gases are the most stable elements and Helium is the most stable element among the noble gases, Helium is considered to have the highest ionisation energy.