Electron Affinity
Chemistry: Cambridge International A Level, topic 1.2.9
Electron Affinity
The electron affinity of an element is the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ/mol.
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Electron affinity vs electronegativity
- The electron affinity of an element is a measurable physical quantity.
- Electron affinity is the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ/mol.
- Electronegativity describes how tightly an atom attracts electrons in a bond.
- It is a dimensionless quantity that is calculated by comparing the amounts of energy required to break different types of bonds.
Trends in electron affinity
- Electron affinity increases from left to right across a period in the periodic table and decreases down a group.
Trends - down a group
- As you go down a group, there is an increased number of electron shells.
- The greater the number of electrons between the nucleus and the outer electrons, the lower the effective nuclear charge.
- This is because the positive charge felt by the electron is reduced by the electrons in between.
- This means that the greater the number of electrons, the lower the electron affinity as it is more difficult to gain an electron.
Trends - across a period
- As you go across a period, the number of protons increases. This means nuclear charge increases across a period.
- As you go across a period, electrons are added to the same shell. For example, all elements in the second period have two electron shells.
- An increased nuclear charge across a period means a greater attraction between the nucleus and the electron to be gained.
- This means electron affinty increases across a period.