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

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 EA₁ will 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