Mulliken defined the electronegativity of an atom as the arithmetic mean of its ionisation energy and electron affinity.
`X_(A)=(1)/(2)(I.P.+E.A.)`
One more relationship given by him, if the values are given in eV is
`X_(A)=("Ionisation potential"+ "Electron affinity")/(5.6)`
When there is pure covalent bond between A-B
`((IP)_(A)+(EA)_(A))/(5.6)=((IP)_(B)+(EA)_(B))/(5.6)`
`implies X_(A)=X_(B)`
According to Mulliken, electronegativity depends on

Options

A. Ionisation potential
B. Electron gain enthalpy
C. Electron affinity
D. Both (1) & (3)

Answer

To solve the question regarding Mulliken's definition of electronegativity and its dependence on certain factors, we can break down the solution into several steps:

### Step-by-Step Solution:

1. **Understanding Mulliken's Definition of Electronegativity**:
- Mulliken defined the electronegativity (X) of an atom as the arithmetic mean of its ionization energy (I.P.) and electron affinity (E.A.). The formula is given as:
\[ X_A = \frac{1}{2}(I.P. + E.A.) \]

2. **Alternative Relationship**:
- An alternative relationship provided by Mulliken states that when values are given in electron volts (eV):
\[ X_A = \frac{I.P. + E.A.}{5.6} \]

3. **Understanding the Implications**:
- The relationship indicates that when there is a pure covalent bond between two atoms A and B, the electronegativity of both atoms is equal:
\[ \frac{(I.P)_A + (E.A)_A}{5.6} = \frac{(I.P)_B + (E.A)_B}{5.6} \implies X_A = X_B \]

4. **Identifying the Factors**:
- From the formula \( X_A = \frac{1}{2}(I.P. + E.A.) \), we can see that electronegativity (X) is directly dependent on both ionization potential (I.P.) and electron affinity (E.A.).
- Ionization potential is the energy required to remove an electron from an atom, while electron affinity is the energy released when an electron is added to an atom.

5. **Conclusion**:
- Therefore, according to Mulliken, electronegativity depends on the ionization potential and electron affinity of the atom.

### Final Answer:
According to Mulliken, electronegativity depends on ionization potential and electron affinity.