Question

Which of the following elements has the highest first ionization enthalpy?

• A. Boron
• B. Carbon
• C. Nitrogen
• D. Oxygen

Hint:

Half-filled (p$^{3}$) and fully-filled (p$^{6}$) subshells show extra stability, leading to higher ionization enthalpy.

Answer 1

The Correct Option is C

Step 1: Recall the meaning of ionization enthalpy.
It is the energy needed to remove the outermost electron from a gaseous atom. A larger value means the electron is held more tightly.

Step 2: Note the period trend.
Across a period from left to right, nuclear charge rises and atoms shrink, so ionization enthalpy generally increases. Boron, carbon, nitrogen, and oxygen sit in this order in period 2.

Step 3: Write the electron configurations.
Boron is $2p^1$, carbon is $2p^2$, nitrogen is $2p^3$, and oxygen is $2p^4$. The $2p^3$ case is special.

Step 4: Use the half-filled stability rule.
Nitrogen has a half-filled $2p^3$ set, which is extra stable. This makes its electron unusually hard to pull out, raising its ionization enthalpy above that of oxygen.

Step 5: Compare the four elements.
So the order rises from boron to carbon, then jumps highest at nitrogen, then drops a little at oxygen. Nitrogen ends up on top.

Step 6: State the answer.
Among boron, carbon, nitrogen, and oxygen, nitrogen has the highest first ionization enthalpy.
\[ \boxed{\text{Nitrogen}} \]