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Electron Affinity

Understanding the energy released when atoms gain electrons

What is Electron Affinity?

Electron Affinity (EA) is the energy released when an electron is added to a gaseous atom. It's the opposite of ionization energy! Higher (more negative) EA means the atom strongly attracts electrons. Units: kJ/mol. Important: EA values can be positive or negative. Negative values = energy released (exothermic, favorable). Positive values = energy required (endothermic, unfavorable).

📊 Factors Affecting Electron Affinity

Four main factors determine how readily an atom gains electrons:

1. Atomic Size

Smaller atoms = Higher EA

Smaller atoms have stronger nuclear attraction for the incoming electron. The new electron comes close to the nucleus, experiencing strong pull.

Example: F has EA = -328 kJ/mol (very favorable), but I has EA = -295 kJ/mol (less favorable) because F is smaller

2. Nuclear Charge

More protons = Higher EA

More protons = stronger pull on incoming electron. This generally makes adding an electron more favorable.

Example: Halogens (Group 17) have very high EA values because they're one electron away from stable noble gas configuration

3. Electron Configuration

Stable configs resist electrons

Noble gases and filled orbitals resist gaining electrons. It's energetically unfavorable to add an electron to a stable configuration.

Example: Noble gases have positive EA (unfavorable). Halogens have negative EA (favorable) because they need one more electron for stability

4. Subshell Type

s, p, d electrons matter

Electrons added to different orbitals experience different effective nuclear charges due to shielding and penetration.

Example: p-orbital elements (like halogens) have higher EA than transition metals in the same period

📈 Periodic Trends in Electron Affinity

Trend 1: Across a Period (Left to Right) → EA Becomes More Negative (Higher)
As you move left to right, electron affinity generally becomes more negative (more favorable). Elements on the right need fewer electrons to complete their octet, making electron gain more favorable.
⚠️ Exception 1: N has higher EA than O

N (1s² 2s² 2p³): Half-filled p orbital is stable. Adding an electron creates pairing repulsion (EA = -7 kJ/mol, nearly neutral)
O (1s² 2s² 2p⁴): Adding electron completes p orbital pair (EA = -141 kJ/mol, more favorable)
Why? Half-filled orbitals resist accepting electrons due to pairing repulsion.

⚠️ Exception 2: Be, Mg, noble gases have positive EA

Be (1s² 2s²): Filled s-shell is stable, resists electrons (EA = +240 kJ/mol)
Ne (full shell): Completely stable, adding electron is unfavorable (EA = +1029 kJ/mol)
Why? Stable configurations energetically resist electron gain.

Key Pattern: Halogens (Group 17) have the most negative EA values because they're one electron away from completing their octet. Noble gases have positive EA because they already have complete octets.
Trend 2: Down a Group (Top to Bottom) → EA Becomes Less Negative (Lower)
As you move down a group, electron affinity becomes less negative (less favorable). The atom becomes larger, and the incoming electron is farther from the nucleus, experiencing weaker attraction.
Halogen Period Atomic Size EA (kJ/mol)
Fluorine (F) 2 Small -328
Chlorine (Cl) 3 Medium -349
Bromine (Br) 4 Large -325
Iodine (I) 5 Very Large -295
⚠️ Special Case: Cl > F in EA

F (EA = -328 kJ/mol) is smaller but has strong orbital repulsion
Cl (EA = -349 kJ/mol) is larger but orbital size matches incoming electron better
Why? Size trend is broken because orbital geometry/overlap affects stability. Cl's 3p orbitals accept the electron with less repulsion than F's 2p orbitals.

Key Pattern: Unlike ionization energy, EA trends are less regular. Size matters, but orbital compatibility and electron repulsion also play major roles.
Trend 3: Alkali Metals vs Halogens
Alkali metals have very low (positive) EA values because they have just completed a stable s² orbital. Halogens have very high (negative) EA values because they need just one electron to complete a p⁶ configuration.
Element Type Examples EA Values (kJ/mol) Why?
Alkali Metals (Group 1) Li, Na, K +53 to +77 Just filled s orbital, stable, resist electrons
Halogens (Group 17) F, Cl, Br -295 to -349 Need one more electron for noble gas config
Noble Gases (Group 18) He, Ne, Ar +941 to +3373 Complete shell, extremely stable, highly repel electrons
Pattern: High magnitude negative EA = atom WANTS electrons. Positive EA = atom RESISTS electrons. Think of it as "electron greediness."

🔗 How It Relates to Ionization Energy

Ionization Energy

Energy required to REMOVE electron
→ Unfavorable process
→ Always positive

High IE = atom holds electrons tightly

Electron Affinity

Energy released when ADDING electron
→ Can be favorable or unfavorable
→ Can be positive or negative

Negative EA = atom attracts electrons strongly

Opposite Processes: Halogens have HIGH ionization energy (hold electrons tight) AND HIGH negative EA (attract electrons strongly). This makes them extremely reactive!
Prediction Tool: Elements with high EA (negative values) will form anions easily. Elements with low IE will form cations easily. This determines chemical bonding!

🔬 Why This Matters