Electron gain enthalpy and ionisation energy of an atom are `-a` and `+b` eV respectively. The electronegativity of that atom on Mulliken scale is given by

Options

A. `a-b`
B. `(b-a)/(2)`
C. `a+b`
D. `(a+b)/(2)`

Answer

To find the electronegativity of an atom on the Mulliken scale, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Definitions**:
- **Electron Gain Enthalpy (EGE)**: This is the energy change when an electron is added to a neutral atom. It is given as `-a eV`, indicating that energy is released when an electron is gained.
- **Ionization Energy (IE)**: This is the energy required to remove an electron from a neutral atom. It is given as `+b eV`, indicating that energy must be supplied to remove an electron.

2. **Mulliken Electronegativity Formula**:
- The electronegativity on the Mulliken scale is defined as the average of the ionization energy and the electron gain enthalpy:
\[ \text{Electronegativity} = \frac{\text{Ionization Energy} + \text{Electron Gain Enthalpy}}{2} \]

3. **Substituting the Values**:
- Substitute the given values into the formula:
\[ \text{Electronegativity} = \frac{(+b) + (-a)}{2} \]
- This simplifies to:
\[ \text{Electronegativity} = \frac{b - a}{2} \]

4. **Final Expression**:
- Therefore, the electronegativity of the atom on the Mulliken scale is:
\[ \text{Electronegativity} = \frac{b + a}{2} \]

### Conclusion:
The electronegativity of the atom on the Mulliken scale is given by:
\[ \text{Electronegativity} = \frac{b + a}{2} \]