Ionization energy: The amount of energy required to remove an electron from an isolated atom or molecule.

Trends down group :

• Ionization energy decreases down the group.
• This is due to the shielding or screen effect of the outer electrons from the nucleus.
• The attraction is weaker and they are more easily removed.
• Helium and radon both belong to group 8 which are called noble gases.
• Helium (He) has the highest ionization energy and Radon (Rn) has the lowest ionization energy.