Periodic Properties of the Elements

Electronic Affinities

Atoms can also gain electrons to form negatively charged ions (anions)

The electron affinity is the energy change associated with an atom or ion in the gas state gaining an electron.

• For all positively charged ions, and most neutral atoms, energy is released when an electron is added

Cl(g) + e^- -> Cl^-(g) DE = -328 kJ/mol

Thus, we say that chlorine has an electron affinity of -328 kJ/mol.

For anions and some neutral atoms, added an electron is an endothermic process, i.e. work must be done to force an electron onto the atom. This results in the formation of an unstable anion.

• The halogens, which are one electron short of a completely filled p subshell have the greatest attraction for an electron (i.e. the electron affinity has the largest negative magnitude)
  
  In adding an electron they achieve a stable electron configuration like the noble gases
• The 2A and 8A groups have filled subshells (s, and p, respectively) and therefore, an additional electron must reside in a higher energy orbital. Adding an electron to these groups is an endothermic process

Electron affinities do not change much as we move down a group

• The distance from the nucleus is increasing with greater n (less attraction)
  however,
• The electrons in the subshell are more diffuse, reducing electron-electron repulsions

1996 Michael Blaber