Name the element which has the highest ionization energy?

Ionization energy:

The minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom.

Factors influencing ionization energy:

Atomic size:

• Ionization energy decreases with increasing atomic size

Nuclear charge:

• Ionization energy increases with increasing nuclear charge

Conclusion:

• Helium (He) is very small and is very close to the nucleus.
• Helium have completely filled orbital.
• Therefore Helium has highest Ionization energy