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Within chemistry, in the study of periodic properties we find electron affinity. This refers to the energy change that occurs when an atom accepts electrons in the ground gas state. This energy change is quantified and reflects the ability of said atom to accept more or fewer electrons. Atoms with a stronger effective nuclear charge have a higher electron affinity.
The reaction that occurs when an atom gains an electron can be represented as follows:
X + e – → X – + Energy
Another way to define electron affinity is as the amount of energy required to remove an electron from a negative ion :
X – + E → X + e –
Key points
- 1.- Electron affinity is the energy change that occurs when an atom accepts electrons in the fundamental gaseous state.
- 2.- It is indicated by the symbol of Ea and is expressed in units of kJ/mol.
- 3.- Electron affinity follows a trend in the periodic table. It increases by moving up a column or group and also increases by moving left to right across a row or period (except for the noble gases).
- 4.- The value can be positive or negative. A negative electron affinity means that energy must be supplied in order to bind an electron to the atom. Here, the capture of electrons is an endothermic process. If the electron affinity is positive, the process is exothermic and occurs spontaneously.
Trend in electron affinity
Electron affinity presents a trend that can be observed in the organization of the elements in the periodic table.
- The electron affinity is greater the higher an element is in a group (column of the periodic table).
- Electron affinity increases from left to right across a period (row of the periodic table). The exception is the noble gases, which are in the last column of the table.
This is due to the fact that as you go down the table, the atomic radius of the elements increases and they have a lower electronic affinity or less energy to release.
Nonmetals typically have higher electron affinity values than metals. Chlorine strongly attracts electrons, while mercury is the element with atoms that most weakly attracts an electron. The affinity for electrons is more difficult to predict in molecules because their electronic structure is more complicated.
Uses of Electron Affinity
Note that the electron affinity values only apply to gaseous atoms and molecules, because the energy levels of liquid and solid electrons are altered by interaction with other atoms and molecules. Even so, electron affinity has various practical applications.
It is used to measure chemical hardness and to predict electronic chemical potential. The primary use of electron affinity values is to determine whether an atom or molecule will act as an electron acceptor or an electron donor. We can also know how the elements will react, if they will produce energy or need it. This information is very useful for predicting results during an investigation.
Signs in electron affinity
Electron affinity is most often represented in units of kilojoules per mole (kJ/mol). Sometimes the values are given in terms of magnitudes relative to each other.
If the value of electron affinity or E ea is negative, it means that energy is needed to fix an electron. Negative values are observed for the nitrogen atom and also for most second electron captures. It can also be seen for substances such as diamond. For a negative value, electron capture is an endothermic process:
E ea = -ΔE (attach)
The same equation applies if E ea has a positive value. In this situation, the change ΔE has a negative value and indicates an exothermic process. Electron capture for most gas atoms (except noble gases) releases energy and is exothermic. One way to remember the capture of an electron having a negative ΔE is to remember the energy it was released.
Remember: ΔE and Eea have opposite signs!
Example electron affinity calculation
The electron affinity of hydrogen is ΔH in the reaction:
H (g) + e – → H – (g); ΔH = -73 kJ/mol, so the electron affinity of hydrogen is 73 kJ/mol. The plus sign is not quoted, however, so E ea is simply written as 73 kJ/mol.
Sources
- Anslyn, Eric V.; Dougherty, Dennis A. (2006). Modern physical organic chemistry . University books Sciences. ISBN 978-1-891389-31-3.
- Atkins, Peter; Jones, Loretta (2010). Chemical Principles for the Search for Insight . Freeman, New York. ISBN 978-1-4292-1955-6.
- Express Classroom (2018) CHEMISTRY. What is Electron Affinity? Periodic table. AULAEXPRESS Baccalaureate. Available at https://www.youtube.com/watch?v=uAyXJ182RzQ&ab_channel=AulaExpress
- Himpsel, F.; Knapp, J.; Vanvechten, J.; Eastman, D. (1979). “Quantum photoyield of diamond (111)-A stable emitter-negative affinity”. Physical Review B. 20(2):624. doi: 10.1103/PhysRevB.20.624
- Tro, Nivaldo J. (2008). Chemistry: A Molecular Approach (2nd Ed.). New Jersey: Pearson Prentice Hall. ISBN 0-13-100065-9.
- IUPAC (1997). Compendium of Chemical Terminology ( 2nd Ed.) (The “Golden Book”). doi: 10.1351/goldbook.E01977
