Electron affinity (EA) represents a fundamental atomic property, quantifying the energy change that occurs when a neutral atom gains an electron. This value provides a direct measure of an atom’s intrinsic tendency to acquire an extra electron and form an anion. Understanding electron affinity is central to predicting how elements will interact, particularly their capacity to form ionic compounds. Determining this precise energy value helps chemists understand the forces that drive chemical reactions and the stability of resulting negative ions.
Defining Electron Affinity
Electron affinity is formally defined as the energy change, often expressed in kilojoules per mole (kJ/mol) or electron volts (eV), when a single electron is added to a neutral atom in the gaseous state. The process is represented by the equation \(X(g) + e^- \to X^-(g)\). This definition specifies that the atom must be isolated and in its gaseous form, ensuring the measurement is an inherent property of the atom itself, free from the influence of neighboring atoms.
For most atoms, the addition of the first electron is an exothermic process, meaning energy is released as the electron is attracted to the atom’s nucleus. In standard thermodynamic terms, an energy release corresponds to a negative change in energy (\(\Delta E\)), but electron affinity tables often list the magnitude of this released energy as a positive value.
The formation of a singly charged anion typically involves an energy release. However, adding a second electron to an already negatively charged ion, as in \(O^-(g) + e^- \to O^{2-}(g)\), is always an endothermic process. Energy must be supplied to overcome the significant electrostatic repulsion between the negatively charged ion and the approaching electron.
Direct Experimental Measurement
The most accurate and direct modern method for determining atomic and molecular electron affinities is Photodetachment Spectroscopy (PDS), which is essentially the reverse process of electron addition. This technique relies on the principle that the energy required to remove an electron from a gaseous negative ion is exactly equal to the electron affinity of the corresponding neutral atom. The experiment begins by generating a beam of the gaseous negative ions.
These ions are then directed into a chamber where they are crossed with a monochromatic laser beam, meaning the light has a precisely known energy (photon energy). When a photon strikes the anion, it can “photodetach” the extra electron, neutralizing the atom according to the reaction \(X^-(g) + \text{photon} \to X(g) + e^-\). The minimum photon energy required to detach the electron is known as the photodetachment threshold.
By systematically varying the laser’s photon energy, scientists can precisely identify the minimum energy needed to observe the first detached electrons. This threshold energy directly corresponds to the electron affinity of the neutral atom. The PDS method is effective because it measures the energy difference between the well-defined ground states of the anion and the neutral atom, providing a direct spectroscopic determination of the electron affinity value.
Indirect Calculation Using Thermodynamics
For elements that do not readily form stable, isolated gaseous anions, electron affinity is frequently determined indirectly using thermochemical principles. This calculation relies on Hess’s Law, applied specifically through the Born-Haber cycle. This cycle is a theoretical path used to relate the lattice energy of an ionic compound to other measurable energy quantities.
The Born-Haber cycle starts with the solid elements in their standard states and traces the energy steps required to convert them into gaseous ions, which then combine to form the ionic crystal lattice. The cycle involves several measurable steps: the enthalpy of sublimation or atomization for the metal, the bond dissociation energy for the non-metal, and the ionization energy for the metal. All these values are experimentally known or can be measured with high accuracy.
The electron affinity of the non-metal is included as the final unknown energy term. By setting the sum of all energy changes in the cycle equal to the measurable standard enthalpy of formation of the ionic compound, the electron affinity can be algebraically isolated and calculated. This technique is especially useful for elements that exist primarily in ionic solids, providing values that complement the direct spectroscopic measurements.
Factors Governing Electron Affinity Values
The magnitude and sign of an atom’s electron affinity are fundamentally governed by three major atomic properties: effective nuclear charge, atomic size, and electron configuration.
The effective nuclear charge, which is the net positive charge experienced by the incoming electron, plays a significant role. A higher effective nuclear charge creates a stronger attractive force on the added electron, leading to a greater release of energy and a larger (more positive) electron affinity value.
Atomic size is inversely related to electron affinity, as the distance between the nucleus and the incoming electron is a critical factor. In smaller atoms, the valence shell is closer to the nucleus, resulting in a stronger attraction for the extra electron and a higher electron affinity. Conversely, in larger atoms, the incoming electron is further away and shielded by more inner-shell electrons, which reduces the attraction.
The electron configuration of an atom introduces specific exceptions to these general trends. Atoms that have a particularly stable configuration, such as a completely filled or half-filled subshell, show a reduced tendency to accept an additional electron. For instance, noble gases and Group 2 elements (alkaline earth metals) have filled subshells, making the addition of an electron energetically unfavorable, resulting in EA values that are close to zero or even negative (endothermic). The halogens, in contrast, are one electron short of a filled shell and consequently exhibit the highest electron affinities in their respective periods.