Electron Affinity

Hand a lone atom one extra electron in the gas phase and measure the energy that flows — that energy change is the electron affinity. It tells you how favorable it is for an isolated gaseous atom to accept one more electron. The one condition to never forget: electron affinity is defined for a gaseous atom, not for an atom already locked in a bond, a solution, or a solid.

The defining process and its symbols:

Here $\text{X}(g)$ is a neutral gas-phase atom and $\text{X}^-(g)$ is the gaseous anion it becomes. If this process releases energy, the first electron affinity is favorable. Some tables report that released energy as a positive number; others report the process's energy change as negative. That clash of conventions is why sign matters more here than almost anywhere else.

Why the Definition Holds Together

Electron affinity is about adding an electron to a single, isolated atom in the gas phase, which is what makes it a clean periodic property — it strips away bonding, solvent, and lattice effects that complicate real reactions. Two related ideas share the same ingredients but answer different questions, so the definition deliberately rules them out:

and electronegativity, which describes how strongly an atom pulls shared electrons inside a bond rather than acquiring a free one.

Worked Example: Chlorine

Chlorine is a strong example because a neutral chlorine atom usually releases energy on gaining one electron:

Why favorable? Chlorine's valence configuration is $3s^2 3p^5$. Gaining one electron gives $3s^2 3p^6$, filling the $3p$ subshell and making the isolated anion lower in energy than the neutral atom alone would suggest. This does not guarantee chlorine gains an electron in every real setting — only that for the isolated gas-phase atom, the one-electron addition is energetically favorable.

Check Your Understanding

1. Predict whether a neutral sodium atom's first electron affinity is more or less favorable than chlorine's, and explain using the periodic trend. Answer check: sodium sits far to the left, so its first electron affinity is much less favorable than chlorine's — gaining an electron does not fill or near-fill a subshell the way it does for chlorine.
2. Write the equation for the second electron affinity of oxygen and state whether it is favorable. Answer check: $\text{O}^-(g) + e^- \rightarrow \text{O}^{2-}(g)$ — adding an electron to an already-negative ion is repelled, so this step is much less favorable than the first.

First vs Second Electron Affinity

The first electron affinity adds one electron to a neutral atom, $\text{X}(g) + e^- \rightarrow \text{X}^-(g)$. The second adds an electron to an anion, $\text{X}^-(g) + e^- \rightarrow \text{X}^{2-}(g)$. These are not similar steps: the second is usually much less favorable because the incoming electron is repelled by a species that is already negatively charged.

The Periodic Trend

First electron affinity generally becomes more favorable across a period from left to right, and often less favorable down a group, though the pattern is not perfectly smooth. It is a guide, not a law: atomic size, subshell structure, and electron-electron repulsion bend it. A standard example is that chlorine's first electron affinity is slightly more favorable than fluorine's, even though fluorine sits above chlorine in the same group.

Calculation and Interpretation Traps

Confusing Electron Affinity With Electronegativity

Electron affinity is an energy change for an isolated atom gaining an electron; electronegativity is an atom's tendency to pull shared electrons in a bond. Related ideas, different questions.

Confusing It With Ionization Energy

Ionization energy removes an electron, $\text{X}(g) \rightarrow \text{X}^+(g) + e^-$. Electron affinity goes the other way — an electron is added, not removed.

Ignoring the Sign Convention

If one source lists favorable electron affinity as positive and another as negative, the numbers look contradictory even for the same chemistry. Always check how the table defines the sign before comparing.

Assuming the Second Electron Works the Same Way

Adding an electron to a neutral atom and to an anion are not equivalent steps. The second addition is usually much less favorable for an isolated species.

A Fast Self-Check

Ask two questions: (1) Am I talking about a neutral gaseous atom gaining one electron? (2) Is my source using released-energy values or signed energy-change values? Clear answers to both usually make the concept easy to interpret.

When Electron Affinity Is Useful

It is useful for comparing periodic trends, explaining why some atoms form anions more readily than others, and separating this idea from electronegativity and ionization energy. In introductory chemistry it builds intuition without pretending every atom follows a perfect trend.