The ascending order of the first ionisation potential
of C, N, O and F is _______.

Answer

To determine the ascending order of the first ionization potential of carbon (C), nitrogen (N), oxygen (O), and fluorine (F), we can follow these steps:

### Step 1: Understand Ionization Potential
The first ionization potential (or ionization energy) is the energy required to remove the outermost electron from an atom. Generally, ionization energy increases across a period in the periodic table due to increased nuclear charge and decreased atomic size.

**Hint:** Remember that ionization energy increases from left to right across a period.

### Step 2: Analyze the Position of Elements
Carbon (C), nitrogen (N), oxygen (O), and fluorine (F) are all in the second period of the periodic table:
- Carbon (C): 1s² 2s² 2p²
- Nitrogen (N): 1s² 2s² 2p³
- Oxygen (O): 1s² 2s² 2p⁴
- Fluorine (F): 1s² 2s² 2p⁵

**Hint:** Look at the number of electrons in the outermost shell for each element.

### Step 3: Compare Ionization Energies
- **Fluorine (F)** has the highest ionization energy because it has the most protons attracting the outermost electrons and is close to having a full outer shell.
- **Oxygen (O)** has a slightly lower ionization energy than fluorine because it has one more electron than nitrogen, which increases electron-electron repulsion.
- **Nitrogen (N)** has a half-filled p subshell (2p³), which is more stable, thus requiring more energy to remove an electron compared to oxygen.
- **Carbon (C)** has the lowest ionization energy of the four because it has fewer protons and a larger atomic radius compared to the others.

**Hint:** Remember that half-filled and fully filled subshells are more stable and require more energy to remove an electron.

### Step 4: Establish the Order
Based on the analysis:
- Fluorine (F) > Nitrogen (N) > Oxygen (O) > Carbon (C)

Thus, the ascending order of the first ionization potential is:
**C < O < N < F**

**Final Answer:** The ascending order of the first ionization potential of C, N, O, and F is C < O < N < F.