Electron Affinity Calculator

Fluorine is the most electronegative element but has a lower EA (3.401 eV) than chlorine (3.613 eV) because fluorine's small size (2p orbitals) causes significant electron-electron repulsion when a second electron is added to the already-crowded 2p subshell. Chlorine's larger 3p orbitals have less repulsion, allowing the added electron to be more stable.

Noble gases (He, Ne, Ar, Kr, Xe) have completely filled valence shells. Adding an electron forces it into the next higher shell (n+1), which experiences less nuclear attraction and greater shielding. The added electron is less stable than a free electron, so energy must be supplied — negative EA. Noble gas anions (if they exist at all) are unstable and very short-lived.

Alkaline earth metals (Be, Mg, Ca) have completely filled s subshells. Adding an electron forces it into a higher-energy p subshell, which is not particularly stabilized by the nuclear charge. The added electron in the p orbital is shielded by the filled s shell. This makes anion formation unfavorable or only marginally favorable for these elements.

Mulliken's electronegativity is defined as the average of ionization energy and electron affinity: chi_M = (IE + EA) / 2. Elements with high IE and high EA (like fluorine and chlorine) have high electronegativity. Elements with low IE and low EA (like cesium) have low electronegativity. Mulliken's scale gives values proportional to, but numerically different from, the more widely used Pauling scale.

Photodetachment spectroscopy measures EA by irradiating a beam of negative ions with tunable laser light. When photon energy exceeds EA, the electron is ejected. The threshold (onset of detachment) gives EA directly. Measuring the kinetic energy spectrum of ejected electrons provides more detailed information about the neutral atom's electronic states.

The second EA involves adding a second electron: X- + e- → X^2-. This is always endothermic (negative) because the negative ion X- repels the incoming electron. For example, the first EA of oxygen is +1.46 eV, but the second EA is -7.28 eV. Despite this, O^2- exists in ionic crystals (like MgO) because the lattice energy stabilizes it.

Across Period 3: Na (0.548) → Mg (-0.38) → Al (0.441) → Si (1.389) → P (0.747) → S (2.077) → Cl (3.613) → Ar (-1.0). Notable: Mg has negative EA (full 3s shell), N and P have lower EA than expected (half-filled p subshell is particularly stable), and halogens have the highest EA in each period.

Nitrogen has the electron configuration [He] 2s^2 2p^3 — a half-filled 2p subshell. By Hund's rule, each 2p orbital is singly occupied with parallel spins. Adding a fourth electron forces it to pair in an already-occupied 2p orbital, causing significant electron-electron repulsion. This makes nitrogen's EA essentially zero (experimentally -0.07 eV — just barely unstable as N-).

High-EA elements are strong oxidizing agents. Fluorine (EA 3.40 eV) and chlorine (EA 3.61 eV) are powerful oxidizers used in industrial processes, water treatment, and as rocket oxidizers (fluorine compounds). Oxygen (EA 1.46 eV) is the principal industrial oxidizer. In electrochemistry, high-EA species readily accept electrons at cathodes, making them useful in batteries and fuel cells.