Animated Solution for Chemistry - Periodicity in Properties: The formation of the oxide ion $O^{2-}(g)$ requires first an exothermic and then an endothermic step as shown below $O(g)+e^{-}=O^{-}(g);\Delta H^{\circ }=-142{\text{ kJ mol}}^{-1}$ $O^{-}(g)+e^{-}=O^{2-}(g);\Delta H^{\circ }=844{\text{ kJ mol}}^{-1}$ This is because

Why Gaining an Electron Costs Energy

Imagine you are observing a single, isolated oxygen atom floating in a vacuum. We want to turn this neutral atom into an oxide ion ($O^{2-}$), which carries a charge of $-2$. You might intuitively think this happens instantly, like a single jump. But in reality, it is a fascinating two-step journey. The atom must capture these electrons one by one, and the physics governing each step is drastically different.

The Welcoming Atom (Exothermic)

Let's look at the very first step. Here, our neutral oxygen atom welcomes its first incoming electron. Oxygen is highly electronegative, meaning it has a strong hunger for electrons to complete its outermost shell and achieve a stable octet.

When an electron approaches, the positively charged nucleus pulls it in eagerly. Because the atom is moving to a more stable state, it releases energy into the surroundings. This is why the first electron gain enthalpy is negative, making it a highly exothermic process. The atom is happy, and energy is given off.

The Hostile Ion (Endothermic)

Now, we move to the second step, and this is where things get really interesting. We no longer have a neutral atom. We now have an $O^{-}$ ion, which already carries a negative charge. And what are we trying to do? We are trying to force a second electron, which is also negatively charged, into this ion.

Think about the fundamental laws of physics here. What happens when you bring two negative charges close together? They repel each other. The $O^{-}$ ion strongly resists the addition of another electron due to intense inter-electronic repulsion. The incoming electron is pushed away by the electron cloud that is already present.

Electrostatic Repulsion

To overcome this massive repulsive force and successfully force the second electron into the shell, we cannot rely on the atom to do it naturally. We have to actively supply a large amount of energy from the outside.

Because we have to pump in so much energy to overcome that electrostatic repulsion, the second electron gain enthalpy becomes highly positive. The process is strongly endothermic. The energy cost of fighting the repulsion completely outweighs any stability the atom might gain by achieving a noble gas configuration.

Therefore, the correct reason for the endothermic nature of the second step is that the $O^{-}$ ion tends to resist the addition of another electron. This is not a phenomenon unique to oxygen. It is a universal trend across the periodic table. Whenever you form a polyvalent anion (like $S^{2-}$ or $N^{3-}$), the first electron might be welcomed, but every subsequent electron will face fierce repulsion, making the subsequent electron gain enthalpies strictly positive.