Question

Which of the following has the lowest ionization potential?
A. Oxygen
B. Nitrogen
C. Sulphur
D. Fluorine

Answer 1

Hint: Ionization potential is the energy required for the removal of an electron from the outermost shell of an isolated gaseous atom. The removal of an electron from the half filled and fully filled configurations is always higher because its difficult to take
out the electron from stable electronic configurations.

Complete answer:
Ionization potential is defined as the energy required for the removal of an electron from the outermost shell of an isolated gaseous atom. It is also known as ionization energy.
On going left to right in a period, the ionization potential increases.
The reason for increases in ionization potential is as follows:
Increase in effective nuclear charge
Decreases in size.
Due to both of these factors, (increase in effective nuclear charge and decreases in size) the stabilization of the outermost shell increases, so it becomes hard to remove an electron from the outermost shell on going left to right in a period.
On going down in a group, the ionization potential decreases.
The reason for decreases in ionization potential is as follows:
Decrease in effective nuclear charge
Increases in size.
Due to both of these factors, (decrease in effective nuclear charge and increases in size) the stabilization of the outermost shell decreases, so it becomes easy to remove an electron from the outermost shell on going down in a group.
Nitrogen is a group-fifteenth element, oxygen and sulphur are the group-sixteenth element and fluorine is a group -seventeenth element.
So, the ionization potential of nitrogen should be lowest. But the ionization potential of nitrogen is highest among all four because the nitrogen has a half-filled electronic configuration which is a very stable electronic configuration.
Down in a group ionization potential decreases so, the ionization potential of sulphur is lower than the oxygen.
So, the decreasing order of ionization potential of all four elements is as follows:
Nitrogen > Fluorine > Oxygen > Sulphur
So, sulphur has the lowest ionization potential among all four.

Therefore, option (C) sulphur is correct.

Note: On going left to right in a period, the electrons are added in the same shell and the proton number also increases, so the attraction of the nucleus on the outermost shell increases. So, the size of the atom decreases. On going down in a group, the electrons get added in the next higher shell so, the attraction of the nucleus on the outermost shell decreases so, the size of the atom increases.