Electron affinity measures an atom’s desire to acquire new electrons. The central question of whether this process releases or absorbs energy—whether it is exothermic or endothermic—is answered by examining the precise circumstances under which an electron is added to an atom. While the general rule leans one way, specific atomic structures create important exceptions.
Defining Electron Affinity and Energy Change
Electron affinity (EA) is defined as the energy change that results when an electron is added to a neutral atom in the gaseous state to form a negative ion, known as an anion. This process is represented by the equation X(g) + e- → X-(g). The sign of the energy change, often denoted as ΔH (enthalpy change), determines whether the process is endothermic or exothermic.
A process is termed exothermic if it releases energy into the surroundings, represented by a negative value for ΔH. Conversely, a process is endothermic if it absorbs energy from the surroundings, requiring an input, and is represented by a positive ΔH value. By convention, the energy value listed as electron affinity is often the magnitude of the energy released, meaning that a positive listed value corresponds to an exothermic process (negative ΔH).
The General Rule: Why Electron Gain Releases Energy
The first electron affinity, which involves adding an electron to a neutral atom, is generally an exothermic process for most non-metal elements. This means that energy is released when the atom gains an electron, resulting in a negative energy change (ΔH). This energy release occurs because the incoming, negatively charged electron is attracted to the positively charged nucleus of the neutral atom.
This electrostatic attraction between the nucleus and the new electron is the driving force that releases energy. As the electron enters the valence shell, the system moves to a more stable, lower energy state. Elements close to having a full outer electron shell, such as the Halogens (Group 17), exhibit a particularly large release of energy. Gaining a single electron allows these atoms to achieve a highly stable, noble gas configuration, making the process significantly exothermic.
Key Exceptions: When Electron Gain Requires Energy
While the first electron affinity is often exothermic, two major scenarios result in an endothermic process, requiring an input of energy. The first involves adding a second or subsequent electron to an already negatively charged ion. The second electron affinity is defined by the reaction X-(g) + e- → X2-(g).
This second process is always endothermic because the incoming electron must overcome the strong electrostatic repulsion from the existing negative charge of the anion. For instance, forming the oxide ion O2- requires a substantial energy input in the second step (O- → O2-) to overcome this repulsion.
Another exception occurs with atoms that already possess a stable electronic configuration, such as the Noble Gases (Group 18) or the Alkaline Earth Metals (Group 2). Adding an electron to these atoms disrupts their inherent stability, forcing the electron into a higher energy level or orbital, which requires energy and makes the process endothermic.
Periodic Trends and Predicting Electron Affinity
The magnitude and sign of electron affinity are directly influenced by an element’s position on the periodic table, allowing for general predictions. Moving from left to right across a period, the electron affinity generally becomes more exothermic (more negative ΔH). This trend is due to the increasing effective nuclear charge, which creates a stronger attractive force for the incoming electron.
Conversely, moving down a group, the electron affinity generally becomes less exothermic. This decrease is caused by the increasing atomic size. The incoming electron is added to a shell further away from the nucleus, weakening the attractive force and releasing less energy.