Electron Affinity Trends
Electron affinity measures the energy change when a neutral atom in the gas phase gains an electron to form a negative ion. A large negative value (often expressed as a large positive value in magnitude) indicates that the atom strongly attracts an additional electron. Understanding electron affinity trends across the periodic table reveals why certain elements readily form anions while others resist gaining electrons entirely.
Defining Electron Affinity
The first electron affinity (EA₁) is the enthalpy change for the process:
X(g) + e⁻ → X⁻(g)
When energy is released (exothermic), the convention assigns a negative value to the enthalpy change, though many textbooks report the magnitude as a positive number. Chlorine, for example, has an electron affinity of approximately −349 kJ/mol, meaning it releases 349 kJ of energy per mole when each chlorine atom captures one electron. This makes chlorine one of the most electron-hungry elements in the periodic table.
Trends Across a Period
Moving from left to right across a period, electron affinity generally becomes more negative (larger in magnitude). Atoms gain protons in their nuclei without adding new electron shells, so the effective nuclear charge increases. The incoming electron feels a stronger pull toward the nucleus, and the atom releases more energy upon capturing it.
In Period 3, sodium has an electron affinity of about −53 kJ/mol, while chlorine reaches −349 kJ/mol. The trend is not perfectly smooth, however — notable exceptions interrupt the pattern.
Trends Down a Group
Moving down a group, electron affinity generally becomes less negative (smaller in magnitude). Although nuclear charge increases, each successive element adds a new electron shell, pushing the outermost electrons farther from the nucleus. The incoming electron lands in a larger, more diffuse orbital and experiences greater shielding from inner electrons. Fluorine has an electron affinity of −328 kJ/mol, while iodine drops to −295 kJ/mol.
Fluorine presents an interesting exception to the group trend. Despite being at the top of Group 17, its electron affinity is actually less negative than chlorine's. The fluorine atom is so small that the incoming electron encounters significant electron-electron repulsion in the compact 2p subshell, partially offsetting the strong nuclear attraction.
Notable Exceptions
Noble gases have essentially zero or positive electron affinities. Their outermost electron shells are completely filled, so an additional electron would have to enter a higher energy level with poor nuclear shielding. There is no energetic advantage to capturing that electron.
Group 2 elements (Be, Mg, Ca) also have very low or positive electron affinities. Their outermost s subshell is already filled (ns²), so an extra electron must enter a higher-energy p orbital. The configuration offers little stabilization.
Nitrogen has a surprisingly low electron affinity compared to its neighbors carbon and oxygen. Nitrogen's three 2p electrons occupy three separate orbitals with parallel spins (a half-filled subshell), which is an especially stable arrangement. Adding a fourth electron forces pairing in one of those orbitals, introducing repulsion and reducing the net energy gain.
Electron Affinity vs. Ionization Energy
Ionization energy measures how difficult it is to remove an electron from an atom; electron affinity measures how favorable it is to add one. Both reflect an atom's grip on electrons, but from opposite directions. Elements with high ionization energies and highly negative electron affinities — the halogens — are the most aggressive electron acceptors, which is why they form stable anions so readily.
Metals, by contrast, have low ionization energies and weakly negative (or positive) electron affinities. They prefer to lose electrons rather than gain them, forming cations in ionic compounds.
Second Electron Affinities
Adding a second electron to an already negative ion is always endothermic because the incoming electron must overcome repulsion from the net negative charge. Oxygen's second electron affinity is about +744 kJ/mol. The oxide ion (O²⁻) exists in ionic crystals only because the lattice energy of the crystal compensates for this unfavorable second addition.
Practical Significance
Electron affinity data help predict which elements form stable anions, the strength of ionic bonds, and the direction of electron transfer in redox reactions. In materials science, electron affinities influence the behavior of semiconductors and the design of electron-transport layers in organic electronics. In atmospheric chemistry, the high electron affinity of oxygen explains why O⁻ ions form readily in the upper atmosphere, affecting radio wave propagation.
Understanding electron affinity alongside ionization energy, electronegativity, and atomic radius provides a comprehensive picture of how atoms interact — forming the chemical bonds that build every material in the world around us.