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?EA_1?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