Electron affinity
The key idea
Electron affinity describes the energy change associated with adding an electron to a gaseous atom or species, with sign conventions requiring careful attention.
Why it matters
Across the periodic table it reflects a competition between nuclear attraction, electron repulsion and the structure of available states.
A picture to remember
Across the periodic table it reflects a competition between nuclear attraction, electron repulsion and the structure of available states.
Convention used on Mendeleev
Mendeleev uses the IUPAC/NIST convention: electron affinity EA is the energy released when an electron is attached to the neutral gaseous species. For the attachment reaction X(g) + e⁻ → X⁻(g), the reaction enthalpy is therefore ΔH = −EA. A positive EA means the anion is lower in energy than the separated neutral species and electron.
Some textbooks instead quote the attachment enthalpy itself. Its sign is opposite. To avoid ambiguity, Mendeleev never mixes these two conventions.