Which of the followig is incorrect

Options

A. Electron gain enthalpy becomes more negative with increase in atomic number across a period
B. effective nuclear charge increase from left to right across period
C. electron gain enthalpy becomes less negative as we go down in a group
D. all of the above

Answer

To determine which of the statements is incorrect, we need to analyze each statement regarding electro-gain enthalpy and effective nuclear charge.

### Step-by-Step Solution:

1. **Understanding Electro-gain Enthalpy**:
- Electro-gain enthalpy is the energy change when an electron is added to a neutral atom in the gas phase.
- A more negative value indicates that energy is released when an electron is added, making the atom more stable.

2. **Electro-gain Enthalpy Trend Across a Period**:
- As we move from left to right across a period in the periodic table, the effective nuclear charge (Z_eff) increases. This is because the number of protons increases while the shielding effect remains relatively constant.
- Therefore, the electro-gain enthalpy becomes more negative (i.e., increases) as it is easier for the atom to attract an additional electron due to the higher nuclear charge.

3. **Electro-gain Enthalpy Trend Down a Group**:
- As we move down a group, the size of the atoms increases due to the addition of electron shells.
- The increased distance between the nucleus and the valence electrons results in a weaker attraction for the incoming electron, making the electro-gain enthalpy less negative (i.e., it decreases).

4. **Effective Nuclear Charge Trend**:
- The effective nuclear charge increases from left to right across a period due to the increase in the number of protons in the nucleus.
- This statement is true as the protons exert a stronger pull on the electrons.

5. **Identifying the Incorrect Statement**:
- The statements analyzed are:
- Electro-gain enthalpy increases across a period (True).
- Effective nuclear charge increases from left to right (True).
- Electro-gain enthalpy is less negative as we go down the group (True).
- Since all statements provided are true, we conclude that there is no incorrect statement among those analyzed.

### Conclusion:
After analyzing the statements, we find that all the provided statements regarding electro-gain enthalpy and effective nuclear charge are correct. Therefore, if the question asked for an incorrect statement, it implies that none of the statements listed are incorrect.