Main Difference – Electron Affinity vs Ionization Energy
Electrons are subatomic particles of atoms. There are many chemical concepts to explain the behavior of electrons. Electron affinity and ionization energy are two such concepts in chemistry. Electron affinity is the amount of energy released when a neutral atom or molecule gain an electron. Electron affinity can be also known as electron gain enthalpy when the meaning is considered, but they are different terms since electron gain enthalpy describes the amount of energy absorbed by the surrounding when an atom gains an electron. Ionization energy, on the other hand, is the amount of energy required to remove an electron from an atom. The main difference between electron affinity and ionization energy is that electron affinity gives the amount of energy released when an atom gains an electron whereas ionization energy is the amount of energy required to remove an electron from an atom.
Key Areas Covered
1. What is Electron Affinity
– Definition, Endothermic and Exothermic Reactions
2. What is Ionization Energy
– Definition, First Ionization, Second Ionization
3. What are the Similarities Between Electron Affinity and Ionization Energy
– Outline of Common Features
4. What is the Difference Between Electron Affinity and Ionization Energy
– Comparison of Key Differences
Key Terms: Atom, Electron, Electron Affinity, Electron Gain Enthalpy, First Ionization Energy, Ionization Energy, Second Ionization Energy
What is Electron Affinity
Electron affinity is the amount of energy released when a neutral atom or a molecule (in the gaseous phase) gains an electron from outside. This electron addition causes the formation of a negatively charged chemical species. This can be represented by symbols as follows.
X + e– → X– + energy
The addition of an electron to a neutral atom or a molecule releases energy. This is called an exothermic reaction. This reaction results in a negative ion. But if another electron is going to be added to this negative ion, energy should be given in order to proceed with that reaction. This is because the incoming electron is repelled by the other electrons. This phenomenon is called an endothermic reaction.
Therefore, the first electron affinities are negative values and the second electron affinity values of the same species are positive values.
First Electron Affinity: X(g) + e– → X(g)–
Second Electron Affinity: X (g)– + e– → X (g)-2
Electron affinity shows periodic variation in the periodic table. This is because the incoming electron is added to the outermost orbital of an atom. The elements of the periodic table are arranged according to the ascending order of their atomic number. When the atomic number increases, the number of electrons they have in their outermost orbitals increases.
Figure 1: Variation of Electron Affinity along a Period of Periodic Table
In general, electron affinity should increase along the period from left to right because the number of electrons increases along a period; thus, it is difficult to add a new electron. When experimentally analyzed, the electron affinity values show a zig-zag pattern rather than a pattern that shows a gradual increase.
What is Ionization Energy
Ionization energy is the amount of energy needed by a gaseous atom in order to remove an electron from its outermost orbital. This is called the ionization energy because the atom gets a positive charge after the removal of an electron and becomes a positively charged ion. Each and every chemical element has a specific ionization energy value since atoms of one element are different from atoms of another element. For example, the first and second ionization energies describe the amount of energy required by an atom to remove one electron and another electron, respectively.
First Ionization Energy
First ionization energy is the amount of energy required by a gaseous, neutral atom to remove its outermost electron. This outermost electron is located in the outermost orbital of an atom. Therefore, this electron has the highest energy among other electrons of that atom. Hence, the first ionization energy is the energy required to discharge the highest energy electron from an atom. This reaction is essentially an endothermic reaction.
This concept is associated with a neutrally charged atom since neutrally charged atoms are composed only of the original number of electrons that the element should be composed of. However, the energy required for this purpose depends on the type of element. If all electrons are paired in an atom, it requires a higher energy. If there is an unpaired electron, it requires a lower energy. However, the value also depends on some other facts. For example, if the atomic radius is high, a low amount of energy is required since the outermost electron is located far from the nucleus. Then the attraction force between this electron and the nucleus is low. Therefore, it can easily be removed. But if the atomic radius is low, then the electron is highly attracted to the nucleus and it is hard to remove the electron from the atom.
Figure 2: Pattern of Varying First Ionizing Energies of Some Chemical Elements
Second Ionization Energy
Second ionization energy can be defined as the amount of energy required to remove an outermost electron from a gaseous, positively charged atom. Removal of an electron from a neutrally charged atom results in a positive charge. This is because there aren’t enough electrons to neutralize the positive charge of the nucleus. Removing another electron from this positively charged atom will require a very high energy. This amount of energy is called the second ionization energy.
Second ionization energy is always a higher value than the first ionization energy since it is very difficult to remove an electron from a positively charged atom than from a neutrally charged atom; this is because the rest of the electrons are highly attracted by the nucleus after removing one electron from a neutral atom.
Similarities Between Electron Affinity and Ionization Energy
- Both are energy-related terms.
- The value of both electron affinity and ionization energy depends on the electron configuration of the subjected atom.
- Both show a pattern in the periodic table.
Difference Between Electron Affinity and Ionization Energy
Definition
Electron Affinity: Electron affinity is the amount of energy released when a neutral atom or molecule (in the gaseous phase) gains an electron from outside.
Ionization Energy: Ionization energy is the amount of energy needed by a gaseous atom in order to remove an electron from its outermost orbital.
Energy
Electron Affinity: Electron affinity describes the release of energy to the surrounding.
Ionization Energy: Ionization energy describes the absorption of energy from outside.
Electron Energy
Electron Affinity: Electron affinity is used to describe electron gaining.
Ionization Energy: Ionization energy is used to describe electron removing.
Conclusion
Electron affinity and ionization energy are two chemical terms used to describe the behavior of electrons and atom quantitatively. The main difference between electron affinity and ionization energy is that electron affinity gives the amount of energy released when an atom gains an electron whereas ionization energy is the amount of energy required to remove an electron from an atom.
Reference:
1. “Electron Affinity.” Chemistry LibreTexts, Libretexts, 14 Nov. 2017, Available here.
2. Electron Affinity, Chem Guide, Available here.
3. Helmenstine, Anne Marie. “Ionization Energy Definition and Trend.” ThoughtCo, Feb. 10, 2017, Available here.
Image Courtesy:
1. “Electron affinities of the elements” By Sandbh – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “First Ionization Energy” By Sponk (PNG file)Glrx (SVG file)Wylve (zh-Hans, zh-Hant)Palosirkka (fi)Michel Djerzinski (vi)TFerenczy (cz)Obsuser (sr-EC, sr-EL, hr, bs, sh)DePiep (elements 104–108)Bob Saint Clar (fr)Shizhao (zh-Hans)Wiki LIC (es)Agung karjono (id)Szaszicska (hu) – Own work based on: Erste Ionisierungsenergie PSE color coded.png by Sponk (CC BY 3.0) via Commons Wikimedia
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