Chemistry · Periodic Classification Of Properties · NEET
The first electron is added to a neutral O atom. The positive nucleus attracts this incoming electron, so energy is released — first electron gain enthalpy is negative (exothermic). The second electron is added to O⁻, which is already negative. Now a negative electron is being forced toward a negative ion. They repel each other, so energy must be SUPPLIED. That makes the second value positive (endothermic).
Positive electron gain enthalpy means energy is ABSORBED (endothermic). The system must take in energy to happen. Negative means energy is RELEASED (exothermic). So O + e⁻ → O⁻ releases energy (negative), but O⁻ + e⁻ → O²⁻ absorbs energy (positive).
Making the free O²⁻ ion needs energy overall. But in a solid ionic compound, the huge lattice energy released when O²⁻ and metal cations pack tightly into a crystal more than pays back that cost. So O²⁻ is stable inside the lattice, not as a lone free ion.
It is positive for essentially all elements. The second electron always has to be added to an already negative ion (like O⁻, S⁻, Cl⁻). Adding a negative electron to a negative ion always faces repulsion, so the second electron gain enthalpy is always positive (endothermic), whatever the element.
O²⁻ does exist — but only where lattice energy or hydration energy pays for it. NCERT wants you to know that the ISOLATED gas-phase step O⁻(g) + e⁻ → O²⁻(g) is endothermic. The final compound is stable for a different reason (crystal lattice), not because the second electron was easy to add.
Ionization enthalpy is the energy to REMOVE an electron (always positive). Electron gain enthalpy is the energy change when you ADD an electron. The first one added can release energy (negative), but the second one added to a now-negative ion needs energy (positive).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is positive. Energy must be absorbed to force a second electron onto the already negative O⁻ ion.
Because a negatively charged electron is being added to a negatively charged ion (O⁻). Like charges repel, so you must supply energy — this makes the process endothermic (positive).
It is about +780 kJ/mol (a positive, endothermic value), while the first is about -141 kJ/mol. NEET mainly tests the SIGN, so remember: first negative, second positive.
In ionic solids the large lattice energy released on crystal formation more than compensates for the energy cost of making O²⁻, so the overall compound is stable.
Yes. The second electron gain enthalpy of sulphur (S⁻ + e⁻ → S²⁻) is also positive, for the same reason: repulsion between the electron and the negative ion.
