Electron Affinity

Electron affinity... is defined as the energy process in which an electron is acquired by the atom in the gas phase

More info... The larger the affinity of an atom for an electron, the more negative the value Electron affinity trends are related to the trends of ionization energy because both represent the energy involved in the gain or loss of an electron by an atom, respectively

The trend down the column... Electron affinity decreases as it goes down a column

This is because... Electrons are added increasingly farther from the nucleus, so the attractive force between the nucleus and electrons decreases The outer electrons experience less of a positive charge as you go up the periodic table (effective nuclear charge)

The trend across a period... Electron affinity increases as it goes across a period

This is because... The octet rule states that atoms with close to full valence shells will tend to gain electrons. When they gain the electrons, they use energy, which allows the ion to become more stable The outer electrons experience less of a positive charge as you go across a period (effective nuclear charge)

Units of EA KJ/Mol ** electron affinity is negative

Exceptions... Nitrogen atoms have no affinity - in N- ions, electron-electron repulsion makes these ions unstable Beryllium anion has no affinity - Be- is not stable, because the added electron is assigned to a higher energy subshell (2p) than the valence electrons (2s)

More Exceptions... Noble gases have no affinity - they have full valence shells and by adding or taking away electrons, they become unstable Fluorine has a low affinity - because it has a small amount of shells and when you add another electron, electron-electron repulsion happens, creating an unstable ion