Question

Correct increasing order of first ionization enthalpy is: C, O, N, F

• A. C < N < O < F
• B. C < O < N < F
• C. O < C < N < F
• D. C < O < F < N

Hint:

Half-filled orbitals are extra stable → higher ionization energy.

Answer 1

The Correct Option is B

Step 1: Understand ionization enthalpy.
Ionization enthalpy is the energy needed to pull out the most loosely held electron from a gaseous atom. A higher value means the electron is harder to remove.

Step 2: Recall the general trend across a period.
As we move left to right in a period, the nuclear charge grows but electrons fill the same shell. The stronger pull holds electrons tighter, so ionization enthalpy mostly rises across the period.

Step 3: Place C, N, O, F in their period.
These four elements all sit in period 2, in the order $C < N < O < F$ by atomic number. So the simple trend would suggest the same rising order of ionization enthalpy.

Step 4: Check for the special stability exception.
Nitrogen has a half-filled $2p^3$ set of orbitals, which is extra stable. This makes nitrogen hold its electron unusually tightly, so its ionization enthalpy is higher than that of oxygen, which sits just after it.

Step 5: Fix the order using the exception.
Because nitrogen jumps above oxygen, the rising order becomes $C < O < N < F$. Carbon stays lowest and fluorine stays highest.

Step 6: State the final increasing order.
So the correct increasing order of first ionization enthalpy is carbon, then oxygen, then nitrogen, then fluorine.
\[ \boxed{C < O < N < F} \]