Electron Affinity Definition

Electron Affinity Definition, Trend, and Example

Electron affinity is a measure of how well an atom can accept an electron.
Electron affinity is a measure of how well an atom can accept an electron. Tommy Flynn, Getty Images

Electron Affinity Definition

Electron affinity reflects the ability of an atom to accept an electron. It is the energy change that occurs when an electron is added to a gaseous atom. Atoms with stronger effective nuclear charge have greater electron affinity.

The reaction that occurs when an atom takes an electron may be represented as:

X + e → X + energy

Another way to define electron affinity is as the amount of energy needed to remove an electron from a singly charged negative ion:

X → X + e

Electron Affinity Trend

Electron affinity is one of the trends that can be predicted using the organization of elements in the periodic table.

  • Electron affinity increases moving down an element group (periodic table column).
  • Electron affinity generally increases moving left to right across an element period (periodic table row). The exception is the noble gases, which are in the last column of the table. Each of these elements has a completely filled valence electron shell and an electron affinity approaching zero.

Nonmetals typically have higher electron affinity values than metals. Chlorine strongly attracts electrons. Mercury is the element with atoms that most weakly attract an electron. Electron affinity is more difficult to predict in molecules because their electronic structure is more complicated.

Uses of Electron Affinity

Keep in mind, electron affinity values only apply to gaseous atoms and molecules because the electron energy levels of liquids and solids are altered by interaction with other atoms and molecules. Even so, electron affinity has practical applications. It is used to measure chemical hardness, a measure of how charged and readily polarized Lewis acids and bases are. It's also used to predict electronic chemical potential. The primary use of electron affinity values is to determine whether an atom or molecule will act as an electron acceptor or an electron donor and whether a pair of reactants will participate in charge-transfer reactions.

Electron Affinity Sign Convention

Electron affinity is most often reported in units of kilojoule per mole (kJ/mol). Sometimes the values are given in terms of magnitudes relative to each other.

If the value of electron affinity or Eea is negative, it means energy is required to attach an electron. Negative values are seen for the nitrogen atom and also for most captures of second electrons. For a negative value, the electron capture is an endothermic process:

Eea = −ΔE(attach)

The same equation applies if Eea has a positive value. In this situation the change Δhas a negative value and indicates an exothermic process. Electron capture for most gas atoms (except noble gases) releases energy and is exothermic. One way to remember capturing an electron has a negative ΔE is to remember energy is let go or released.

Remember: Δand Eea have opposite signs!

Example Electron Affinity Calculation

The electron affinity of hydrogen is ΔH in the reaction

H(g) + e- → H-(g); ΔH = -73 kJ/mol, so the electron affinity of hydrogen is +73 kJ/mol. The "plus" sign isn't cited, though, so the Eea is simply written as 73 kJ/mol.