• When atoms gain electrons they become negative ions or?anions
• Electron affinity (EA)?can be thought of as the opposite process of ionisation energy and is defined as
  • The amount of energy released when?one mole?of electrons is gained by?one mole?of atoms of an element in the gaseous state to form?one mole?of gaseous ions
• Electron affinities are measured under?standard conditions?which are 298 K and 100 kPa
• The units of EA are?kilojoules?per?mole?(kJ mol^-1)
• The?first electron affinity?is always exothermic
  • E.g. the first electron affinity of chlorine is:

Cl (g) + e^-?→?Cl^-?(g)? ? ? ? ? ? ? ? ??H = - 349 kJ mol^-1

• However, the second electron affinity can be an endothermic process

O^-?(g) + e^-?→?O^2-?(g)? ? ? ? ? ? ? ?H = + 753 kJ mol^-1

• This is due to the fact that you are overcoming repulsion between the electron and a negative ion, so energy is required making the process endothermic overall

Trends in electron affinity

Electron affinities across a period

• Electron affinities show?periodicity
• The pattern is very similar to ionisation energies, except that it is inverted and the minimum points are displaced one element to the right
• As might be expected, the most exothermic electron affinities are for group 17 elements which also have the highest electronegativities
• The strongest pull on electrons correlates with the greater amount of energy released when negative ions are formed
• Noble gases do not form negative ions, so they don't appear in this chart
• The electron affinities reach a peak for group 2 and group 5 elements

Electron affinities down a group

• Electron affinities?generally decrease down a group
• As the atoms become larger the attraction for an additional electron is less, since the?effective nuclear charge?is reduced due to increased?shielding
• Electron affinity become?less exothermic?going down the group
• An exception to this is fluorine whose?electron affinity?is smaller than expected
• This is because fluorine is such a small atom and an additional electron in the 2p subshell experiences considerable repulsion with the other valence electrons