Electron Affinity & Trends of Group 16 & 17 Elements (Cambridge International A-Level Chemistry)

Electron Affinity

Spec point — 23.1.2

• 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

• The value for EA_{1* *}will usually be negative

• 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
  • The values will be positive

Second & third electron affinity summary

• EA₁

  • X (g) + e^– → X^– (g)
  • Exothermic

• EA₂

  • X^– (g) + e^– → X^2– (g)
  • Endothermic

• EA₃

  • X^2– (g) + e^– → X^3– (g)
  • Endothermic

Factors affecting electron affinity

The electron affinity of an element depends on how strongly the nucleus attracts an incoming electron.

• The stronger the attraction between the nucleus and the incoming electron, the more energy is released, making the electron affinity more exothermic (more negative)

• The factors that affect electron affinity are the same as those that influence ionisation energy:

• Nuclear charge

  • A higher nuclear charge means a stronger pull on the incoming electron, resulting in a more exothermic electron affinity

• Distance (atomic radius)

  • A larger distance between the nucleus and the outermost shell reduces the attractive force, making electron affinity less exothermic

• Shielding

  • More inner electron shells increase shielding, which weakens the nuclear attraction for the incoming electron, leading to a less exothermic electron affinity

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 higher nuclear charge than sulfur

  • This stronger nuclear charge results in a greater attraction between the nucleus and the incoming electron
  • Therefore, more energy is released when an electron is added to chlorine, making its first electron affinity (EA₁) more exothermic than that of sulfur

• As you move down Group 16 or Group 17:

  • The outermost electrons are farther from the nucleus, so the attractive force is weaker
  • There are more electron shells, increasing shielding and further reducing nuclear attraction
  • It becomes more difficult to add an electron to the outer shell
  • Less energy is released, so the electron affinity becomes less exothermic

• Fluorine as an exception

  • Fluorine has a very small atomic radius, resulting in:
  • High electron density around the nucleus.
  • Increased repulsion between the incoming electron and the existing electrons.
  • This repulsion weakens the overall attraction to the nucleus.
  • As a result, fluorine’s first electron affinity is less exothermic than expected, and it is actually lower than that of chlorine.