The properties of the elements such as atomic or ionic radii ionisation enthalpy, electron gain enthalpy and electronegativity are directly or indirectly related to their electronic configuration and are called periodic properties. A part the periodic table is given below :
`{:(C, N, O, F),(, P, S, Cl),(,,Se, Br), (,,,I):}`
The highest ionisation enthalpy is of

Options

A. P
B. O
C. N
D. S

Answer

To determine which element has the highest ionization enthalpy among the given elements (C, N, O, F, P, S, Cl, Se, Br, I), we need to analyze their electronic configurations and the periodic trends related to ionization enthalpy.

### Step-by-Step Solution:

1. **Understand Ionization Enthalpy**:
- Ionization enthalpy (or ionization energy) is the energy required to remove an electron from an isolated gaseous atom.
- Generally, ionization energy increases across a period and decreases down a group in the periodic table.

2. **Identify the Elements**:
- The elements in the provided part of the periodic table are:
- Second Period: C (Carbon), N (Nitrogen), O (Oxygen), F (Fluorine)
- Third Period: P (Phosphorus), S (Sulfur), Cl (Chlorine)
- Fourth Period: Se (Selenium), Br (Bromine), I (Iodine)

3. **Analyze the Periodic Trends**:
- **Across a Period**: As you move from left to right across a period, the ionization energy increases due to the increasing nuclear charge which attracts the electrons more strongly.
- **Down a Group**: As you move down a group, the ionization energy decreases because the outermost electrons are farther from the nucleus and are shielded by inner electrons.

4. **Compare the Elements**:
- For the second period (C, N, O, F):
- C has the lowest ionization energy.
- N has a higher ionization energy than C due to its half-filled p subshell (2p3), which provides extra stability.
- O has a higher ionization energy than N, but it is affected by electron-electron repulsion in its p subshell.
- F has the highest ionization energy in the second period due to its high nuclear charge and small atomic radius.
- For the third period (P, S, Cl):
- Cl has a higher ionization energy than S and P.
- For the fourth period (Se, Br, I):
- Br has a higher ionization energy than Se and I.

5. **Conclusion**:
- Among all these elements, **Fluorine (F)** has the highest ionization enthalpy due to its position in the periodic table, being at the far right of the second period.

### Final Answer:
The element with the highest ionization enthalpy is **Fluorine (F)**.

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