# Electron Affinity Calculator

Look up electron affinity values for common elements in kJ/mol and eV. Calculate total energy released when atoms gain electrons.

## What this calculates

Look up electron affinity values for common elements and calculate the energy released (or absorbed) when a neutral atom gains an electron. Results in both kJ/mol and electron volts.

## Inputs

- **Element** — options: Hydrogen (H), Lithium (Li), Boron (B), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), Sodium (Na), Aluminum (Al), Silicon (Si), Phosphorus (P), Sulfur (S), Chlorine (Cl), Potassium (K), Bromine (Br), Iodine (I), Custom Value — Select an element or enter a custom electron affinity.
- **Custom Electron Affinity** (kJ/mol) — Enter a custom first electron affinity value (used when Element = Custom).
- **Moles of Atoms** (mol) — min 0 — Number of moles of atoms gaining an electron.

## Outputs

- **Electron Affinity** (kJ/mol) — First electron affinity of the selected element.
- **Electron Affinity** (eV) — Electron affinity converted to electron volts.
- **Total Energy Released** (kJ) — Energy change for the given number of moles.
- **Details** — formatted as text — Element info and calculation steps.

## Details

Electron affinity (EA) is the energy change when a neutral atom in the gas phase gains one electron to form a negative ion. For most nonmetals, this process releases energy (positive EA in the convention used here).

**The Reaction**

X(g) + e- -> X-(g) + energy

**Sign Convention**

This calculator uses the thermodynamic convention where a positive value means energy is released (exothermic). Some textbooks flip the sign -- always check which convention is in use.

**Periodic Trends**

Electron affinity generally increases across a period (left to right) and decreases down a group, with notable exceptions:

- **Chlorine** has the highest EA (349 kJ/mol), not fluorine, because fluorine's tiny size causes strong electron-electron repulsion
- **Nitrogen** and **phosphorus** have near-zero or negative EA because their half-filled p orbitals are already quite stable
- **Noble gases** have very negative (unfavorable) EA because adding an electron means starting a new shell

**Common Values**

| Element | EA (kJ/mol) |
|---------|------------|
| Cl | 349.0 |
| F | 328.0 |
| Br | 324.6 |
| I | 295.2 |
| S | 200.4 |
| O | 141.0 |
| C | 121.8 |
| H | 72.8 |

## Frequently Asked Questions

**Q: What is electron affinity?**

A: Electron affinity is the energy change when a neutral gaseous atom gains one electron. A positive value (in the thermodynamic convention) means the process is exothermic and the atom readily accepts the electron.

**Q: Why does chlorine have a higher electron affinity than fluorine?**

A: Fluorine is so small that the added electron experiences strong repulsion from the existing electrons crowded into a tiny 2p orbital. Chlorine has a larger 3p orbital with more room, so the added electron is more stable despite being farther from the nucleus.

**Q: Why is nitrogen's electron affinity near zero?**

A: Nitrogen has a half-filled 2p subshell (2p3), which is an especially stable arrangement. Adding a fourth electron means pairing up in an orbital, which costs energy and nearly cancels out the attraction from the nucleus.

**Q: What is the difference between electron affinity and electronegativity?**

A: Electron affinity is the energy change when a free atom gains an electron in the gas phase. Electronegativity measures how strongly an atom attracts electrons within a chemical bond. They follow similar trends but measure different things.

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Source: https://vastcalc.com/calculators/chemistry/electron-affinity
Category: Chemistry
Last updated: 2026-04-08