What is electron affinity?

Short Answer

Electron affinity is the energy change that occurs when an atom in the gaseous state gains an electron to form a negative ion. It indicates how easily an atom accepts an electron.

  • Elements with high electron affinity, like halogens, release more energy when gaining an electron.
  • Elements with low or negative electron affinity, like noble gases, do not readily accept electrons, showing low reactivity.
Detailed Explanation :

Definition of Electron Affinity

Electron affinity (EA) is defined as the energy released or absorbed when a neutral atom in the gaseous state gains an electron to form an anion. It is usually expressed in kilojoules per mole (kJ/mol).

  • Negative EA: Energy is released when the electron is added → favorable process.
  • Positive EA: Energy is required to add an electron → unfavorable process.
  • Electron affinity is a measure of an atom’s tendency to attract electrons.

Process of Electron Gain

  1. Halogen Example: Cl (g) + e⁻ → Cl⁻ (g) ΔE = -349 kJ/mol
    • Energy is released → high electron affinity.
  2. Noble Gas Example: Ne (g) + e⁻ → Ne⁻ (g) ΔE = +48 kJ/mol
    • Energy must be supplied → very low or positive electron affinity.

Factors Affecting Electron Affinity

  1. Atomic Size:
    • Smaller atoms → added electron is closer to nucleus → stronger attraction → higher EA.
  2. Nuclear Charge:
    • More protons → stronger attraction → higher electron affinity.
  3. Electron Configuration:
    • Half-filled and fully filled orbitals → lower EA due to stability.
    • Example: Oxygen (O) has lower EA than fluorine (F).
  4. Shielding Effect:
    • Inner electrons reduce effective nuclear pull → lower EA.

Trend in the Periodic Table

  1. Across a Period (Left to Right):
    • Electron affinity generally increases.
    • Reason: Atomic radius decreases, nuclear charge increases → stronger attraction for the incoming electron.
    • Example: Li → -59 kJ/mol, F → -349 kJ/mol
  2. Down a Group (Top to Bottom):
    • Electron affinity generally decreases.
    • Reason: Added electron is farther from the nucleus due to more shells → weaker attraction.
    • Example: F → -349 kJ/mol, Cl → -349 kJ/mol, Br → -324 kJ/mol, I → -295 kJ/mol

Significance of Electron Affinity

  1. Predicting Reactivity:
    • Non-metals with high EA gain electrons easily → form anions → participate in ionic bonding.
  2. Bond Formation:
    • Determines energy released during formation of ionic compounds.
  3. Periodic Trends:
    • Helps explain trends in electronegativity, ionization energy, and metallic character.
  4. Chemical Behavior:
    • Elements with negative EA tend to form stable negative ions; elements with low or positive EA resist gaining electrons.
  5. Industrial Applications:
    • Used in chemical synthesis, halogen reactions, and semiconductor design.

Examples of Electron Affinity Trends

  • Halogens: High EA → form salts with metals (NaCl, KBr).
  • Alkali Metals: Low EA → do not easily gain electrons.
  • Noble Gases: Positive or very low EA → almost inert.

Special Observations

  1. Oxygen (O) has slightly lower EA than expected due to electron-electron repulsion in the added electron.
  2. Nitrogen (N) also shows lower EA due to half-filled 2p³ configuration → stable.
Conclusion

Electron affinity measures the energy change when an atom gains an electron, showing an atom’s tendency to form negative ions. It increases across a period due to higher nuclear charge and decreases down a group due to added electron shells. Electron affinity is crucial for understanding reactivity, bonding, periodic trends, and chemical properties of elements in the periodic table.