Which elements (Oxygen or Fluorine) have more negative electron gain enthalpy ?

Why Fluorine (F) has a more negative electron gain enthalpy than Oxygen (O) ?

Electron Gain Enthalpy (ΔH_eg)

Electron gain enthalpy is the energy change when a neutral atom in the gaseous state gains an electron to form an anion: X(g) + e⁻ → X⁻(g)

• A more negative value means the atom releases more energy when it gains an electron, so it has a stronger tendency to accept an electron.

Electronic Configurations

• Oxygen: 1s²2s²2p⁴
• Fluorine: 1s²2s²2p⁵

Fluorine has only one electron less than the stable noble gas configuration (2s²2p⁶), while oxygen has two electrons less.

1. Closer to noble gas configuration: Fluorine needs just one electron to achieve the stable octet. This makes the electron gain more favorable, so more energy is released.
2. Smaller size: Fluorine is smaller than oxygen, so the nucleus exerts a stronger attraction on the added electron.
3. Stronger effective nuclear charge (Z_eff): Fluorine’s nucleus attracts the incoming electron more strongly than oxygen’s, releasing more energy.

That’s why F has a more negative electron gain enthalpy than O.

✅ Summary:

Fluorine has a more negative electron gain enthalpy than oxygen because:

(a) It has a 2s²2p⁵ configuration, needing only one electron to complete its octet.

(b) Its small size and high nuclear charge attract the extra electron strongly.