Why is the second electron affinity of an atom usually shown by a positive sign?
Answer (1)
Electron affinity refers to the energy change that occurs when an electron is added to an atom in the gaseous phase. For most atoms, this process is exothermic; that is, energy is released when an electron is added.
However, when we talk about the second electron affinity , we're referring to the energy change that occurs when a second electron is added to an already negatively charged ion.
Here's why the second electron affinity is typically positive:
Repulsion Factor : When the first electron is added to a neutral atom, it generally fits into an available space in the electron cloud, releasing energy in the process. But once an atom becomes an anion (negatively charged), adding another electron means overcoming the electron-electron repulsion, because electrons naturally repel each other.
Energy Requirement : To introduce a second electron into an already negatively charged ion, energy must be supplied to overcome the repulsive force between the extra electron and the negative ion. This makes the second electron affinity an endothermic process, which is why it is positive.
For example, consider the oxygen atom:
The first electron is added to a neutral oxygen atom, releasing energy (exothermic).
Adding a second electron to form O²⁻ requires energy because the already present negative charge repels additional electrons.
In conclusion, the second electron affinity is positive because energy must be inputted to push the second electron onto an already negatively charged ion due to the repulsive forces at play.
