Electron Affinity and Periodic Trends

Electron Affinity

• Definition: The energy change that occurs when an electron is added to a neutral atom to form a negative ion (anion).
• Positive Electron Affinities: Some elements have positive electron affinities, indicating that energy must be supplied to add an electron to the atom. This results in the formation of a negatively charged ion where the number of electrons exceeds the number of protons.

Halogens

• Characteristic: Halogens possess the highest electron affinities among all elements. This property makes them very reactive, as they readily gain electrons to complete their outer electron shell.

Trends in Electron Affinity

• Across Periods:

• General trend: Electron affinity tends to increase across a period from left to right.

• Reason: Atoms on the right side of the periodic table (such as halogens) are more likely to gain an electron to achieve a full outer shell, resulting in higher electron affinity values.

• Down Groups:

• General trend: Electron affinity decreases as you move down a group in the periodic table.

• Example: In Group 1, moving from Lithium to Potassium results in lower electron affinity values at the bottom of the group due to increasing atomic size and electron shielding effects, which reduce the effective attraction of the incoming electron.