Electron Gain Enthalpy

Introduction

Electron gain enthalpy describes the energy change when an electron is added to an isolated gaseous atom. It is useful for interpreting how readily certain atoms form monatomic anions. The sign convention must be read carefully: an energy-releasing addition has a negative enthalpy change.

Core explanation

For a neutral atom X, write X(g) + e⁻ → X⁻(g). The first electron gain enthalpy is the enthalpy change when one mole of gaseous atoms each accepts one electron to form one mole of gaseous anions. If the attraction between the incoming electron and the atom releases more energy than is required to accommodate electron–electron repulsion and orbital rearrangement, ΔH is negative. A positive value means energy must be supplied for that defined gas-phase addition.

The term electron affinity is often used for a closely related quantity, but textbooks do not always use the same sign convention. Some report the energy released as a positive number; others report the process enthalpy as a negative number. To avoid confusion, write the reaction and state whether the number is an enthalpy change or an energy-release magnitude. “More favorable” may mean more negative under the enthalpy convention.

Halogens generally have strongly negative first electron gain enthalpies because adding one electron fills their outer p subshell. For chlorine, Cl(g) + e⁻ → Cl⁻(g) is exothermic. That does not mean a chlorine molecule, Cl₂, accepts an electron by the same defined process, nor does it prove that every reaction involving chlorine is spontaneous. Bond dissociation, ionization of another reagent, lattice formation and solvation may all matter to an overall reaction.

The first electron gain step differs from adding a second electron to an already negative ion. For oxygen, the first step O(g) + e⁻ → O⁻(g) can release energy. The second step O⁻(g) + e⁻ → O²⁻(g) must push an electron toward a negatively charged ion and is endothermic in the gas phase. Oxide ions can nevertheless be stabilized in ionic solids by the energy released when the crystal forms. Isolated-ion energetics and compound stability must not be conflated.

Electron gain enthalpy is not simply the negative of first ionisation enthalpy for the same neutral atom. The latter removes an electron from X(g) to form X⁺(g); the former adds one to form X⁻(g). They involve different final species and electron configurations. The reverse of X(g) + e⁻ → X⁻(g) is electron removal from X⁻, not first ionisation of neutral X.

Nuclear attraction, atomic size, orbital occupancy and electron repulsion all influence electron gain values. A small atom can attract an incoming electron strongly, yet its compact orbital can create unusually high repulsion. That balance produces important exceptions to a simple periodic arrow.

Step-by-step reasoning

1. Write X(g) + e⁻ → X⁻(g) for the first addition. 2. Check that the starting species is a neutral gaseous atom. 3. Decide whether net energy is released or absorbed. 4. Assign a negative enthalpy change to energy release. 5. Separate this atomic step from any later solid or solution formation.

Visual explanation

Draw a gaseous chlorine atom with seven outer dots. Add one incoming electron arrow and show Cl⁻ with a filled outer arrangement. Put a downward energy arrow beside the process. In a second sketch, point an electron toward O⁻ and label repulsion for the second addition.

Real-world analogy

An empty seat near a strong central attraction may welcome one extra person, but pushing another into an already crowded, negatively charged region is harder. The analogy captures the difference between first and second additions without treating electrons as literal people or seats.

Real-world example

A Born–Haber cycle for sodium chloride includes chlorine's gas-phase electron-gain step. The overall formation of NaCl(s) also includes sodium ionisation, chlorine bond splitting and crystal formation. Using the isolated electron-gain value alone would miss most of the energy story.

Why?

Why insist on a gaseous atom? It isolates electron attachment from chemical bonds, crystal forces and solvation. That makes the value a comparable atomic property.

Common misconception

“A positive electron affinity value always means the electron-gain enthalpy is positive.” Some sources define affinity as energy released, with the opposite sign. Always read the stated convention and reaction.

Worked example

Suppose adding an electron to gaseous X releases 300 kJ per mole. For X(g) + e⁻ → X⁻(g), the electron gain enthalpy is −300 kJ mol⁻¹ under the reaction-enthalpy convention. A table that defines affinity as the released-energy magnitude may instead print +300 kJ mol⁻¹. The physical process is the same; the sign label differs.

Quick check

1. What sign does an exothermic first electron-gain enthalpy have? Answer: Negative under the reaction-enthalpy convention.

Exam focus

Include gas-state symbols, one incoming electron and a −1 gaseous ion. Define the sign explicitly, especially if a question uses “electron affinity.” Keep first and second electron additions separate.

Advanced insight

Gas-phase electron attachment can be measured through energy-resolved experiments or inferred through thermochemical cycles. A bound anion may have multiple electronic states, so the reported value refers to a specified process and state. This is another reason to identify the chemical equation rather than memorize only a number.

Summary

First electron gain enthalpy is ΔH for X(g) + e⁻ → X⁻(g). It is negative when energy is released and positive when energy is required. The quantity differs from ionisation enthalpy, a second electron addition and the overall energy of making an ionic compound.

Practice questions

1. Write the first electron-gain process for fluorine. Answer: F(g) + e⁻ → F⁻(g). 2. If electron attachment releases 250 kJ mol⁻¹, what is ΔH under the enthalpy convention? Answer: −250 kJ mol⁻¹. 3. Why can an oxide-containing solid exist if O⁻(g) + e⁻ → O²⁻(g) costs energy? Answer: Other steps, especially formation of a strongly bound ionic lattice, can release enough energy to stabilize the compound overall.