How does the electron gain enthalpies vary across a period and down a group ?

Options

A. More positive across period and less positive down a group
B. More negative across a period and less negative down a group
C. Less positive across a period and less negative down a group
D. More positive across a period and less negative down a group

Explanation

To understand how electron gain enthalpy varies across a period and down a group, we need to analyze the concept of electron gain enthalpy and the trends in the periodic table.

### Step-by-Step Solution:

1. **Definition of Electron Gain Enthalpy**:
- Electron gain enthalpy is defined as the amount of energy released when an electron is added to a neutral gaseous atom. It is usually expressed in kJ/mol.

2. **Variation Across a Period**:
- As we move from left to right across a period in the periodic table, the electron gain enthalpy generally becomes more negative (i.e., the value increases in magnitude). This means that atoms become more favorable in gaining an electron.
- This trend occurs because the effective nuclear charge increases as protons are added to the nucleus while the number of shielding electrons remains relatively constant. As a result, the attraction between the nucleus and the added electron increases, leading to a greater release of energy.

3. **Variation Down a Group**:
- When moving down a group in the periodic table, the electron gain enthalpy generally becomes less negative (i.e., the value decreases in magnitude). This indicates that it becomes less favorable for atoms to gain an electron.
- This trend can be attributed to the increase in atomic size and the greater distance between the nucleus and the added electron. The increased shielding effect from the inner electron shells also reduces the effective nuclear charge experienced by the added electron, making it less energetically favorable to gain an electron.

4. **Exceptions to the Trend**:
- It is important to note that there are exceptions to these trends. For example, noble gases have positive electron gain enthalpies because they have a complete valence shell and do not favor gaining an electron.

5. **Conclusion**:
- In summary, electron gain enthalpy becomes more negative across a period due to increasing nuclear charge and less negative down a group due to increased atomic size and shielding effect.