On the Mulliken scale, the average of ionization potential and electron affinity is known as electronegativity. In essence, the Mulliken electronegativity scale is defined by the formula:

$$ \chi_{Mulliken} = \frac{I + E}{2} $$

Where:

• $$ \chi_{Mulliken} $$ is the Mulliken electronegativity,
• $$ I $$ is the ionization potential (the energy required to remove an electron from an atom in the gaseous state),
• $$ E $$ is the electron affinity (the energy released when an electron is added to a neutral atom in the gaseous state).

This formula essentially provides a measure of how strongly an atom in a molecule attracts electrons towards itself. The concept of electronegativity is crucial in understanding chemical bonding, particularly the polarity of bonds and molecular structures. By using the average of ionization potential and electron affinity, Mulliken's approach provides a quantitative way of assessing an atom's ability to attract shared electrons in a chemical bond.