Question

Element	First Ionization Energy (kJ/mol)
Lithium	520
Sodium	496
Rubidium	403
Cesium	376

According to above table, which is most likely to be the first ionization energy for potassium?

• A. $536kJ/mol$
• B. $504kJ/mol$
• C. $419kJ/mol$
• D. $391kJ/mol$
• E. $358kJ/mol$

Answer 1

The correct option is C $419kJ/mol$

According to above table, most likely the first ionization energy for potassium is $419kJ/mol$.
It is in between the first ionization energies for $Na$ and $Rb$. On moving down the group, the value of first ionization enthalpy decreases with increase in atomic size due to increase in shielding effect.