Which of the following statements is incorrect?

Options

A. Greater is the nucler charge, greater is the electron gain enthalpy
B. Nitrogen has almost zero electron gain enthalpy
C. electron gain enthalpy decreases from fluorine to iodine in the group
D. Chlorine has highest electron gain enthalpy

Answer

To determine which statement is incorrect among the given options, we need to analyze each statement carefully based on our understanding of periodic properties, particularly focusing on nuclear charge and electron gain enthalpy.

### Step-by-Step Solution:

1. **Understanding Nuclear Charge and Electron Gain Enthalpy**:
- The nuclear charge refers to the total charge of the nucleus due to the presence of protons.
- Electron gain enthalpy is the energy change when an electron is added to a neutral atom in the gaseous state.

2. **Analyzing the First Statement**:
- The first statement claims that "greater the nuclear charge, greater is the electron gain enthalpy."
- As we move from left to right in a period, the nuclear charge increases while the electron shielding remains relatively constant. This leads to a stronger attraction between the nucleus and the incoming electron, thus increasing the electron gain enthalpy.
- **Conclusion**: This statement is correct.

3. **Analyzing the Second Statement**:
- The second statement claims that "nitrogen has almost zero electron gain enthalpy."
- Nitrogen has a half-filled p subshell (2s² 2p³), which is a stable configuration. Atoms with stable configurations tend to have lower tendencies to gain additional electrons, resulting in a near-zero electron gain enthalpy.
- **Conclusion**: This statement is correct.

4. **Analyzing the Third Statement**:
- The third statement discusses the trend in the halogen group, specifically that "as we go down the group, electron gain enthalpy decreases."
- Typically, as we move down a group, atomic size increases, leading to a decrease in electron gain enthalpy. However, there is an exception between fluorine and chlorine where fluorine has a higher (less negative) electron gain enthalpy than chlorine due to increased electron-electron repulsion in the small 2p subshell of fluorine.
- **Conclusion**: This statement is incorrect because it does not account for the exception between fluorine and chlorine.

5. **Analyzing the Fourth Statement**:
- The fourth statement claims that "chlorine has the highest electron gain enthalpy."
- Chlorine does have a high electron gain enthalpy, but due to the exception noted earlier, it is important to clarify that while chlorine's electron gain enthalpy is high, it is not the highest when compared to fluorine in terms of absolute values; however, it is more negative than that of fluorine.
- **Conclusion**: This statement is correct.

### Final Conclusion:
The incorrect statement among the options is the **third statement**, which fails to recognize the exception in the trend of electron gain enthalpy between fluorine and chlorine.