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Wize University Chemistry Textbook > Periodic Table Trends

Electron Affinity Trend

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Electron Affinity (EA)

Electron affinity is the amount of energy involved with adding an electron to an atom or ion.

Wize Tip
Don't confuse ionization energy (IE) with electron affinity (EA)!
  • Ionization energy is when we remove the outermost electron
  • Electron affinity is when we add an electron
Memory tip: Electron affinity has an "a" in it for adding an electron

When adding an electron, do we get a cation or an anion?
anion
!

E+e−→E−ΔE=Electron Affinity\textbf{E}+\textbf{e}^-\rightarrow \textbf{E}^-\quad\quad\Delta \textbf{E}=\textbf{Electron Affinity}

  • If EA is negative → energy is released/absorbed:
    released
  • If this is the case, would the atom go to a higher/lower E state?
    lower
  • This means the atom has become more/less
    more
    stable as a result of adding an electron
  • Did this atom like or dislike having an electron added to it?
    like
  • If EA is positive → energy is released/absorbed:
    absorbed
  • If this is the case, would the atom go to a higher/lower E state?
    higher
  • This means the atom has become more/less stable
    less
    stable as a result of adding an electron
  • Did this atom like or dislike having an electron added to it?
    dislike

Watch Out!
Use a negative EA as a reference point, unless the question states otherwise.

  • This means that unless specified otherwise, when asked which element would have the largest electron affinity, assume they mean largest NEGATIVE electron affinity.
  • This would mean that the element really liked getting an electron and lost energy, resulting in a more stable form.

The Periodic Trend

In general, as Zeff increases, the incoming electron experiences a greater electrostatic attraction and stabilization leading to a greater (negative) electron affinity


What about noble gases?

Do you think a noble gas would have a positive or negative electron affinity?
Positive
  • Are noble gases stable?
    Yes!
  • Would a noble gas want another electron added?
    No!
  • If you tried to add an electron to a noble gas would that require energy or release energy?
    Require energy

Watch Out!
This trend excludes noble gases. Noble gases have stable, completely filled shells. Adding electrons to noble gases will break the noble gas configuration.


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Example: Increasing Electron Affinity

Rank the following atoms in increasing absolute energy of their first electron affinity: Br, Ag, Ba, Mo, Sb


Ba < Mo < Ag < Sb < Br


Practice: Positive Electron Affinity

For most elements, the first electron affinity is negative; however, a few elements have a positive first electron affinity. Which atom has the most positive first electron affinity?

Practice: Highest Electron Affinity

Which of the following has the highest electron affinity?
Extra Practice
Electron Affinity, periodic trend
Choose the element with the more exothermic electron affinity from each pair:
1. S, Mg; 2. B, S; 3. Si, P; 4. Br, Ga.
Electron Affinity
Choose the element with more exothermic EA from each pair
Electron Affinity
Which of the following species releases the least energy when an electron is added to it?
Electron Affinity
Which of the following atoms will have a positive electron affinity.
Electron Affinity
Which of the following atoms will have a positive electron affinity.
Periodic Trends: Electron Affinity
For most elements, the first electron affinity is negative; however, a few elements have a positive first electron affinity. Which of the following atoms would have a positive first electron affinity?
Periodic Trends: Electron Affinity
Why is the electron affinity of P less negative than either Si or S? That is, why is adding an electron to a neutral phosphorous (P) atom less exothermic (less favourable) than adding an electron to a silicon or
sulphur atom?
Periodic Trends: Electron affinity
The trend of electron affinity on the periodic table is to from top to bottom and
from left to right
Electron Affinity
Which of the following has the lowest electron affinity?
"The Periodic Table", image retrieved from Chemistry 2e (https://d3bxy9euw4e147.cloudfront.net/oscms-prodcms/media/documents/Chemistry2e-OP_QCzV58S.pdf) by Flowers, P et al., is licensed under CC BY.
Which of the following has the highest electron affinity?
N N+ N-
"The Periodic Table", image retrieved from Chemistry 2e (https://d3bxy9euw4e147.cloudfront.net/oscms-prodcms/media/documents/Chemistry2e-OP_QCzV58S.pdf) by Flowers, P et al., is licensed under CC BY.
Rank the following atoms/ions in order of increasing electron affinity: Cl, Cl-, Mg.
Periodic Trends: Electron. Affinity
Considering the electron affinity of the atoms involved, for which reaction is the electron affinity not exothermic?
A. S(g)+e−→S−(g)S(g)+e^-\rightarrow S^-(g)
B. Si(g)+e−→Si−(g)Si(g)+e^-\rightarrow Si^-(g)
Periodic Trends: Electron Affinity
An exception to the periodic trend for electron affinity, Carbon actually has a higher electron affinity than Nitrogen. This is because:
Periodic Trends: Electron Affinity Exceptions
An exception to the periodic trend for electron affinity, Carbon actually has a higher electron affinity than Nitrogen. This phenomenon is best explained by which of the following statements?
Why does nitrogen have a positive electron affinity? (i.e. How do we rationalize the fact that it is energetically unfavourable for an N atom to accept an electron when it is energetically favourable for C and O to do so?)
Periodic Trends: Electron Affinity
The electron affinity for Ge is -118.94 kJ/mol while the electron affinity for Al is -41.76 kJ/mol. The atomic property that best explains this difference in electron affinity is:
like Winter 2014 MT1
Which equation describes the electron affinity of Bromine?