How can electron affinity be negative?

Question

Cameron Andrews · Accepted Answer

Electron affinity can be negative if the process releases energy.

The process's enthalpy change is equal to the electron affinity: #X_((g))+erarrX_((g))^-# The effective charge of the nucleus, which is less than the actual nuclear charge because of the "shielding" effect of the inner electrons, will attract the incoming electrons. Separating two oppositely charged particles requires effort; on the other hand, bringing two of these particles together releases energy. Think about chlorine: #Cl_((g))+erarrCl_((g))^-# #E=-379.5"kJ"# An incoming electron will go into the outer #n=3# energy level. It can be thought as an #n=oo# to #n=3# transition. This will produce light as photons. The kinetic energy of the electron is released as it slams into the atom, transferring energy to the atom and producing a roughly 50:50 ratio of heat to light. This explains why some reactions, like the burning of sodium in chlorine, can be so amazing. It should be noted that the overall energy cycle determines whether or not the process will be exothermic, and electron affinity is merely one step in it. One example of an endothermic second electron affinity is: #"O_((g))^(-)+erarrO_((g))^(2-)# #E=+623"kJ"# This is because it takes energy to force two negative charges to come together.

Luna Chen · Answer

undefined Electron affinity can be negative when the energy released upon adding an electron to an atom is less than zero, indicating that the atom is more stable after gaining an electron. This occurs when the attraction between the incoming electron and the nucleus is weaker than the repulsion between the added electron and the existing electrons in the atom's outer shells.