CIE A Level Chemistry复习笔记5.1.2 Electron Affinity & Trends of Group 16 & 17 Elements

Electron Affinity

Electron affinity

  • The first electron affinity (EA1) is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms, to form 1 mole of gaseous ions each with a single negative charge under standard conditions

X(g) + e- → X-(g)

  • EA1 is usually exothermic, as energy is released
    • Since this is generally an exothermic process, then the value for EAwill usually be a negative number
  • An element can also accept more than one electron, in which case successive electron affinities are used
    • For example, the second electron affinity (EA2) and third electron affinity (EA3) of an element represent the formation of 1 mole of gaseous ions with 2- and 3- charges respectively
  • The second and third electron affinities are endothermic, as energy is absorbed
    • This is because the incoming electron is added to an already negative ion
    • Energy is required to overcome the repulsive forces between the incoming electron and negative ion
    • Since these are endothermic processes, the values will be positive

Second & third electron affinity table


Factors affecting electron affinity

  • The value of the electron affinity depends on how strongly the incoming electron is attracted to the nucleus
    • The greater the attractive forces between the electron and nucleus, the more energy is released and therefore the more exothermic (more negative) the EA1 value will be
  • The factors affecting the electron affinity of an element are the same as those that affect the ionisation energy (the formation of positive ions via the loss of electrons)
  • These are:
    • Nuclear charge: the greater the nuclear charge, the stronger the attractive forces between an incoming electron and the nucleus
    • Distance: the greater the distance between the nucleus and the outermost shell/orbital where the electron is added, the weaker the force of attraction
    • Shielding: the greater the number of shells, the greater the shielding effect and the weaker the force of attraction

Trends in electron affinity of Group 16 & Group 17 elements

  • Electron affinities of non-metals become more exothermic across a period, with a maximum at Group 17
  • There is generally a downwards trend in the size of the electron affinities of the elements in Group 16 and 17
    • The electron affinities generally become less exothermic for each successive element going down both Groups, apart from the first member of each Group (oxygen and fluorine respectively)

Electron affinity table


  • An atom of chlorine has a greater nuclear charge than an atom of sulfur
  • Chlorine will therefore have a greater attractive force between its nucleus and its outer electrons
    • More energy is released upon adding an electron to chlorine, so the EA1of Cl is more exothermic than for S
  • Going down Group 16 and 17:
    • The outermost electrons are held less tightly to the nucleus as they are further away
    • The number of electron shells increases causing an increased shielding of the outermost electrons
    • It gets more difficult to add an electron to the outer shell
    • Less energy is released upon adding an electron to the outer shell
    • So generally, the EA1becomes less exothermic
  • Fluorine is an exception and has a lower EA1 than chlorine
    • Fluorine has a very small atomic radius
    • This means that the electron density of fluorine is high
    • There is more repulsion between the incoming electron and the electrons that are already present in fluorine
    • These repulsive forces reduce the attractive forces between the incoming electron and nucleus
    • As a result, the EA1 of fluorine is less exothermic than expected