Electron Affinity (EA) and Electronegativity (EN) are often confused, yet they represent two distinct ways of measuring an atom’s tendency to attract electrons. Both properties describe how strongly an atom attracts electrons, but they apply in different contexts and use fundamentally different types of measurement. Understanding the precise definition of each property is essential for clarifying why they are related but not interchangeable.
Defining Electron Affinity
Electron Affinity (EA) is defined as the energy change that occurs when a neutral atom in the gaseous state gains an electron to form a negative ion, or anion. This process is represented by the equation X(g) + e- → X-(g) + energy, where X is the neutral gaseous atom. Since the process of an atom accepting an electron typically releases energy, the first electron affinity is usually an exothermic reaction.
Electron Affinity is an absolute, experimentally measurable quantity, typically expressed in kilojoules per mole (kJ/mol) or electron volts (eV). A highly positive or large magnitude of electron affinity indicates that the atom strongly favors accepting an electron and releasing a large amount of energy upon doing so. This value is quantitative and measurable.
A positive electron affinity signifies that the resulting anion is more stable than the initial neutral atom and the free electron. Conversely, a negative electron affinity means energy must be supplied for the atom to accept the electron, indicating the process is endothermic and the atom resists forming the anion. Electron Affinity is a property of the isolated atom, making it useful for predicting the likelihood of ion formation in the gas phase.
Defining Electronegativity
Electronegativity (EN) is a chemical property that quantifies an atom’s tendency to attract a shared pair of electrons when it is already part of a chemical bond within a molecule. Unlike electron affinity, electronegativity is not a measure of energy released or absorbed. It is instead a relative measure of an atom’s electron-pulling power within a molecular structure.
The most common method for quantifying this property is the Pauling scale, proposed by Linus Pauling. This scale is based on the difference between the actual bond energy of two different atoms and the hypothetical bond energy if the bond were purely covalent. The resulting values are unitless, as they are a comparison rather than an absolute measurement of energy.
The Pauling scale assigns values that range from approximately 0.7 for the least electronegative elements, like Francium, up to 3.98 for the most electronegative element, Fluorine. Because it is a relative scale, it cannot be measured directly but must be calculated from other properties, such as bond energies. Electronegativity is primarily used to predict the polarity of a chemical bond and the resulting distribution of electron density in a molecule.
Key Differences Between the Concepts
The fundamental difference between the two concepts lies in the environment of the atom being considered. Electron Affinity describes the behavior of a neutral atom in isolation, specifically in the gaseous state, as it gains an external electron. In contrast, Electronegativity describes an atom’s behavior when it is chemically bonded to another atom, focusing on its ability to attract the electrons already being shared in that bond.
The nature of their measurement also sets them apart. Electron Affinity is an absolute, measurable thermodynamic quantity of energy. It is a quantitative property, meaning it has specific units like kJ/mol. Electronegativity, conversely, is a relative, unitless value derived from comparisons, which makes it a qualitative property used for ranking elements.
Their primary applications highlight their distinct focus. Electron Affinity is used to predict the stability of gaseous negative ions and the likelihood of their formation. The value of Electron Affinity helps determine if the formation of an anion will be an energy-releasing or energy-requiring process. Electronegativity is instead used to predict the type of chemical bond that will form between two atoms, specifically whether the bond will be nonpolar covalent, polar covalent, or ionic.
Why Their Periodic Trends Are Similar
Despite their definitional differences, Electron Affinity and Electronegativity exhibit similar periodic trends across the periodic table, which is a major source of confusion. Both properties generally increase as one moves from left to right across a period and decrease as one moves down a group. This similarity in trend is not a coincidence; it is rooted in the same fundamental atomic properties that govern all electron-related behavior.
The primary factors influencing both properties are the effective nuclear charge and the atomic radius. Moving across a period, the effective nuclear charge increases because the number of protons in the nucleus increases, while the valence electrons are added to the same electron shell. This greater positive charge exerts a stronger pull on electrons, increasing the atom’s attraction for both an incoming electron (Electron Affinity) and a shared bonding pair (Electronegativity).
Moving down a group, the atomic radius increases as new electron shells are added. The outermost electrons are farther from the nucleus, and the inner electrons shield the valence shell from the full nuclear charge. This shielding effect and greater distance reduce the pull on electrons, leading to a decrease in both the energy released when an electron is gained and the atom’s ability to attract shared electrons. The underlying physics of nuclear attraction and electron shielding drives the parallel behavior observed for both Electron Affinity and Electronegativity.