Animated Solution for Chemistry - Periodicity in Properties: When the first electron gain enthalpy (${\Delta }_{eg}H$) of oxygen is $-141\text{ kJ /mol}$, its second electron gain enthalpy is

Imagine an isolated, neutral oxygen atom floating in a vacuum. It has eight protons in its nucleus and eight electrons orbiting around it. The atom is relatively stable, but it has a deep desire to complete its octet. When a stray electron wanders by, the oxygen atom's nucleus exerts an attractive pull on it.

As the electron falls into the outermost shell, the system moves to a lower energy state. This release of energy is what we call the first electron gain enthalpy (${\Delta }_{eg}{H}_1$). For oxygen, this process is exothermic, releasing $141\text{ kJ/mol}$ of energy. The equation looks like this:

Now, our oxygen atom has transformed into a uni-negative ion, $O^{-}$. It has an extra electron, meaning the overall charge of the species is now negative. The electron cloud has expanded slightly due to inter-electronic repulsion, but the ion is still hungry for one more electron to achieve that perfect noble gas configuration.

However, there is a massive physical barrier. What happens when we try to force a second electron into this already negative ion?

We are bringing a negatively charged electron towards a negatively charged $O^{-}$ ion. Like charges repel! There is a tremendous electrostatic repulsion between the incoming electron and the electron cloud of the $O^{-}$ ion.

To overcome this intense electrostatic repulsion and force the second electron into the shell, we cannot rely on the atom's natural attraction. Instead, we must actively supply a large amount of energy from the outside.

Because energy is absorbed by the system to make this happen, the process is highly endothermic. Therefore, the second electron gain enthalpy (${\Delta }_{eg}{H}_2$) of oxygen is a positive value.

This isn't just a quirky trait of oxygen. For any element in the periodic table, adding a second electron to a negative ion will always face this exact same electrostatic repulsion. Thus, the second electron gain enthalpy is universally positive. Keep this golden rule in mind, as it is a classic trap in competitive exams!