Question

How does ionization energy of sodium $\left(Na\right)$ compare to that of potassium $\left(K\right)$?

• A. The ionization energy of $K$ is less than $Na$ because although there are more protons, $K$ has $4$ electron shells compare to $Na$ which has $3$ electron shells. This causes an increase in nuclear shielding, requiring less energy to remove the electron.
• B. The ionization energy of $K$ is more than $Na$ because there are more protons and one extra electron shell is $K$. This causes an increase in nuclear shielding and requires more energy to remove the electron.
• C. The ionization energy of $K$ is less than $Na$ because there are fewer protons and one less electron subshell. Since there is less attraction, it requires less energy to remove the electron.
• D. The ionization energies of both elements are equal because they are of the same group, the alkaline earth metals.

Answer 1

The correct option is A The ionization energy of $K$ is less than $Na$ because although there are more protons, $K$ has $4$ electron shells compare to $Na$ which has $3$ electron shells. This causes an increase in nuclear shielding, requiring less energy to remove the electron.

The major role in determining IE is played by effective nuclear charge and not the number of protons. The effective nuclear charge decreases due to increasing shielding which in turn reduces the IE. Thus the comparison of IE of $Na$ and $K$ is explained in option A.