The second electron gain enthalpies (in kJ `mol^(-1)`) of oxygen and sulphur respectively are:

Options

A. `-780,+590`
B. `-590,+780`
C. `+590,+780`
D. `+780,+590`

Answer

To determine the second electron gain enthalpies of oxygen and sulfur, we can follow these steps:

### Step 1: Understand Electron Gain Enthalpy
- Electron gain enthalpy is the energy change when an atom gains an electron. For the first electron gain, energy is typically released (negative value). However, for the second electron gain, the situation changes.

### Step 2: Analyze the First Electron Gain
- When oxygen (O) gains its first electron, it forms O⁻. This process releases energy, resulting in a negative electron gain enthalpy. The same applies to sulfur (S) when it gains its first electron to form S⁻.

### Step 3: Consider the Second Electron Gain
- The second electron gain involves an anion (O⁻ or S⁻) gaining another electron. For O⁻ to gain another electron and become O²⁻, it must overcome the repulsion between the negatively charged O⁻ and the incoming electron. This requires energy, making the second electron gain enthalpy positive.

### Step 4: Compare Oxygen and Sulfur
- Both O and S will have positive second electron gain enthalpies. However, we need to compare their values. As we move down the group in the periodic table, the atomic size increases, which generally decreases the tendency to gain electrons.

### Step 5: Analyze the Size and Electron Repulsion
- Oxygen has a smaller atomic size compared to sulfur. The small size of oxygen leads to significant electron-electron repulsions in its 2p subshell when it tries to accommodate a second electron. This means that more energy is required for oxygen to gain a second electron compared to sulfur.

### Step 6: Conclusion
- Therefore, the second electron gain enthalpy of oxygen is higher (more positive) than that of sulfur. The final values are:
- Second electron gain enthalpy of oxygen: approximately +144 kJ/mol
- Second electron gain enthalpy of sulfur: approximately +200 kJ/mol

### Final Answer
The second electron gain enthalpies of oxygen and sulfur are approximately +144 kJ/mol and +200 kJ/mol, respectively.

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