Which of the following elements has the lowest ionization energy?

Na

C

F

He

Rb

Rb

Explanation:

Ionization energy is defined required energy to eliminate or remove an electron from an ion or atom.

From the given elements Rb or Rubidium has the lowest ionization energy as it has lowest shielding effect, so it easy to remove electron from it's shell.

The ionization energy generally decreases from top to bottom in groups due to lower shielding effect and outer electrons are loosely packed so easy to remove and increases from left to right across a period because of valence shell stability.

Rubidium has atomic number 37 and lies below than other given elements and also placed in the left side in the periodic table.

Hence, the correct option is Rb