Question

Which of the following statements best describes the first ionization energies of sodium compared to potassium?
A) The first ionization energy for sodium is greater than the first ionization energy for potassium due to a decrease in the effective nuclear charge.
B) The first ionization energy for sodium is less than the first ionization energy for potassium due to an increase in the effective nuclear charge.
C) The first ionization energy for sodium is greater than the first ionization energy for potassium due to a decrease in nuclear shielding.
D) The first ionization energy for sodium is less than the first ionization energy for potassium due to an increase in nuclear shielding.

Answer 1

Consider the relative positions of sodium and potassium in the periodic table. Consider different factors that affect the ionization energy. These include shielding effect, nuclear charge, and effective nuclear charge.

Complete answer:
When you move from top to bottom in the modern periodic table, you will observe an increase in the nuclear charge and decrease in effective nuclear charge. The nuclear charge increases because the number of protons increases. The effective nuclear charge decreases because the increase in the shielding is more than the increase in the nuclear charge.
With decrease in the effective nuclear charge, the ionization energy will also decrease. This is because you need less energy to remove an electron, as the electron will experience less influence from the nucleus.
In the group of alkali metals, potassium is placed below sodium. Hence, potassium has lower effective charge than sodium. So, potassium has lower first ionization energy than sodium.
Thus, the following statement best describes the first ionization energies of sodium compared to potassium.
The first ionization energy for sodium is greater than the first ionization energy for potassium due to a decrease in the effective nuclear charge.

Hence, the option (A) is the correct option.

Note: The ionization energy is the amount of energy needed to remove an electron from an isolated gaseous atom. When you remove only one electron, you call the ionization energy as first ionization energy. When you remove the second electron, you call the ionization energy as second ionization energy.