Among P,S,CI,F most and least negative `DeltaH_(eg)` will be respectively
of :

Options

A. CI,P
B. P,CI
C. CI,F
D. F,CI

Answer

To determine which element among phosphorus (P), sulfur (S), chlorine (Cl), and fluorine (F) has the most and least negative electron gain enthalpy (ΔH_eg), we can follow these steps:

### Step 1: Understand Electron Gain Enthalpy
Electron gain enthalpy is the energy change when a neutral gaseous atom gains an electron to form an anion. A more negative ΔH_eg indicates that energy is released and the process is more favorable.

### Step 2: Identify the Group and Period of Each Element
- Phosphorus (P) - Group 15, Period 3
- Sulfur (S) - Group 16, Period 3
- Chlorine (Cl) - Group 17, Period 3
- Fluorine (F) - Group 17, Period 2

### Step 3: Analyze Trends in Electron Gain Enthalpy
1. **Across a Period**: As we move from left to right across a period, the electron gain enthalpy becomes more negative. This is because elements are more eager to gain electrons to achieve a stable electron configuration.
2. **Down a Group**: As we move down a group, the electron gain enthalpy becomes less negative. This is due to increased distance from the nucleus and increased electron shielding, making it less favorable to add an electron.

### Step 4: Compare the Elements
- **Fluorine (F)**: Being in the second period, it has a small atomic radius, but it experiences significant inter-electronic repulsion when gaining an electron due to its small 2p orbital.
- **Chlorine (Cl)**: In the third period, it has a larger atomic radius than fluorine, reducing inter-electronic repulsion when gaining an electron, making it more favorable.
- **Sulfur (S)**: Also in the third period, it has a less negative ΔH_eg compared to chlorine.
- **Phosphorus (P)**: Being further left in the periodic table, it has the least negative ΔH_eg among these elements.

### Step 5: Conclusion
- **Most Negative ΔH_eg**: Chlorine (Cl) has the most negative electron gain enthalpy due to its larger size compared to fluorine, allowing for easier electron addition.
- **Least Negative ΔH_eg**: Phosphorus (P) has the least negative electron gain enthalpy because it is further left in the periodic table and less inclined to gain electrons compared to the others.

### Final Answer
- Most negative ΔH_eg: Chlorine (Cl)
- Least negative ΔH_eg: Phosphorus (P)