Electron Gain Enthalpy - Meaning, Definition, Differences, Factors, FAQs

Electron Gain Enthalpy - Meaning, Definition, Differences, Factors, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:31 PM IST

Electron gain enthalpy is the change in energy when an atom or molecule takes up an electron to produce a negatively charged ion. Electron gain enthalpy indicates an element’s ability to accept an additional electron as stated by the energy released or absorbed during an electron gain.

In this article, we will be focussing on the in-depth knowledge of the important topic of electron gain enthalpy, which is the sub-topic of the chapter Classification Of Elements And Periodic table from class 11 chemistry. It is not only essential for board exams but also for competitive exams like the JEE Mains Exam, NEET Exam, and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE, and more.

This Story also Contains
  1. Electron Gain Enthalpy Of Elements
  2. Electron Gain Enthalpy Table
  3. Variation of Electron Gain Enthalpy
  4. Solved Examples Based on- Electron Gain Enthalpy
Electron Gain Enthalpy - Meaning, Definition, Differences, Factors, FAQs
Electron Gain Enthalpy - Meaning, Definition, Differences, Factors, FAQs

Electron Gain Enthalpy Of Elements

Studying electron gain enthalpy reveals the nature of elements’ dealing with electrons, their ability to accept or give away electrons determines their chemical activity. Atoms with low electron affinity lose energy when taking the extra electrons and the more energy is lost the better of forming negative ions. It is vital in explaining trends of periodicity and in making estimations of other chemical reactions in all branches of chemistry.

Electron gain enthalpy

Electron Gain Enthalpy (ΔegH)

Electron gain enthalpy is the energy change that occurs when an electron is added to a neutral gaseous atom to form a negative ion. It is also known as electron affinity. f13
A(g)+e→A−(g)+Δeg H

Factors affecting electron gain enthalpy

The electron gain enthalpy or electron affinity depends upon various factors such as:

  • Atomic Size

With the increase of atomic size, the distance between the nucleus and the last shell electrons also increases due to which the force of attraction between the nucleus and the incoming electron decreases. Hence, the electron gain enthalpy becomes less negative.

  • Nuclear Charge

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With the increase of nuclear charge, the force of attraction between the nucleus and the incoming electron increases. Thus, the electron gain enthalpy becomes more negative.

  • Electronic Configuration

Elements that have half-filled or filled orbitals are more stable than others. In these cases Generally, energy has to be provided to add an electron. Thus, their electron gain enthalpy generally has large positive values.

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Electron Gain Enthalpy Table

Electron Gain enthalpy table

Variation of Electron Gain Enthalpy

  • The electron gain enthalpy becomes less negative in going from top to bottom in a group.

  • In moving from top to bottom in a group, both the atomic size and the nuclear charge increase. However, the effect of the increase in atomic size is more dominant than the nuclear charge.

  • With the increase in atomic size, the attraction of the nucleus for the incoming electron decreases. Hence, the electron gain enthalpy becomes less negative. But in moving from left to right in a period, the attraction of the nucleus and the incoming electron increases, and thus electron gain enthalpy becomes more negative.

  • Halogens have the most negative electron gain enthalpies. In moving down from chlorine to iodine, the electron gain enthalpies become less negative due to the increase in their atomic radii.

  • Chlorine has the most negative electron gain enthalpy value than fluorine. Because fluorine is very small in size due to which there is a very strong inter-electronic repulsion for the incoming electron, thus its electron gain enthalpy is less than chlorine.

  • Generally, Members of the 2nd period in p-block elements show the anomalous value of electron gain enthalpy.

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Solved Examples Based on- Electron Gain Enthalpy

Example 1: Electron affinity is the

1) Energy absorbed when an electron is added to an isolated atom in the gaseous state

2) Energy is released when an electron is added to an isolated atom in the gaseous state

3) The energy required to take out an electron from an isolated gaseous atom

4) The power of an atom to attract an electron to itself

Solution: As we have learned,

Electron affinity is the ability of any isolated gaseous atom to take up a free electron. Energy is released in the process of electron gain.

Hence, the answer is the option (2).

Example 2: The correct order of electron affinity is :

1) F > Cl > O

2) F > O > Cl

3) Cl > F > O

4) O > F > Cl

Solution: As we learned in

Variation of electron gain enthalpy in the periodic table -

  1. Electron gain enthalpy decreases down the group.
  2. generally, the electron gains enthalpy increases (more negative) along the period.

The correct order of electron affinity is Cl > F > O

Hence, the answer is an option (3).

Example 3: Which one of the following arrangements represents the correct order of electron gain enthalpy value (less negative) of the given atomic species?

1) S<O<Cl<F

2) O<S<F<Cl

3) Cl<F<S<O

4) O<S<Cl<F

Solution: As we learned

Variation of electron gain enthalpy with the size of an atom -

In general, the electron gain enthalpy value decreases (less negative) with increasing atomic radius.

- wherein

egH−or E.A ∝ 1 /Atomic radius

Electron gain enthalpy decreases with increasing atomic radius.

Hence, the answer is the option (2).

Example 4: The electron gain enthalpy(in kJ/mol) of fluorine, chlorine, bromine, and iodine respectively are:

1) −333,−349,−325 and -296
2) −349,−333,−325 and -296
3) −296,−325−333, and -349
4) −333,−325−349 and -296

Solution: The electron gain enthalpy becomes less negative in going from top to bottom in a group.

Chlorine has the most negative electron gain enthalpy value than fluorine. Because fluorine is very small in size due to which there is a very strong inter-electronic repulsion for the incoming electron, thus its electron gain enthalpy is less than chlorine.

The electron gain enthalpy values are given below:

Fluorine = -333kJ/mol
Chlorine = -349kJ/mol
Bromine = -325kJ/mol
Iodine = -296kJ/mol

Hence, the answer is the option(1).

Example 5: In which of the following pairs, electron gain enthalpies of constituent elements are nearly identical?
(A) Rb and Cs
(B) Na and K
(C) Ar and Kr
(D) I and At
Choose the correct answer from the options given below :

1) (A) and (B) only

2) (B) and (C) only

3) (A) and (C) only

4) (C) and (D) only

Solution: From the given option -

(Rb and Cs) have nearly the same electron gain enthalpy, nearly -46 KJ/mol.

Whereas (Ar and Kr) have nearly the same electron gain enthalpy is +96KJ/mol.

Hence, the answer is the option (3).

Practice more Questions from the link given below:


Conclusion:

Electron gain enthalpy (ΔₑgH) measures the energy change when a gaseous atom gains an electron: negative if energy is released, positive if absorbed. I becomes more negative across a period, and less negative down a group, with notable exceptions like fluorine versus chlorine due to electronic repulsion. Highly negative values in halogens drive strong anion formation, while positive values in noble gases and stable configurations resist electron gain. This concept is crucial for predicting reactivity, guiding industrial processes (e.g., battery chemistry, water disinfection), and serves as a fundamental principle in competitive exams and materials design.

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Frequently Asked Questions (FAQs)

1. What is the electron affinity of phosphorus?

The electron affinity of phosphorus is 72 KJ/mol.

2. What is the difference between electron gain enthalpy and electronegativity?
  1. The enthalpy change that occurs when a neutral gaseous atom accepts an extra electron to create an anion is known as electron gain enthalpy. It is a quantitative property and can be measured.

The tendency of an element's atom in a chemical compound to attract a shared pair of electrons towards it in a covalent bond is known as electronegativity. It is a qualitative property and hence is not measurable.

3. Which is the element with the highest electron gain enthalpy?

Chlorine has the highest electron gain enthalpy.

4. What is the relationship between electron gain enthalpy and electron affinity?

Electron gain enthalpy = Electron affinity – 52RT, where R is the universal gas constant and T is the temperature in kelvin.

5. What is the relationship between electron gain enthalpy and electron affinity?
Electron gain enthalpy and electron affinity are essentially the same concept, but with a sign convention difference. Electron gain enthalpy is the energy change when an atom gains an electron, with negative values indicating energy release. Electron affinity is traditionally defined as the energy released when an atom gains an electron, so it has the opposite sign of electron gain enthalpy.
6. Name the factors on which electron gain enthalpy depends?
  1. The factors influencing the electron gain enthalpy are :

  • Atomic size                                                

  • Nuclear Charge

  • Electronic configuration

7. How is electron gain enthalpy measured?

 Electron gain enthalpy is measured experimentally by determining the energy change when an electron is added to a neutral atom in the gas phase. This can be done using calorimetry or other thermodynamic methods.

8. Which type of element typically has high (positive) electron gain enthalpy?

Noble gases typically have high (positive) electron gain enthalpy because they have complete valence electron shells and are energetically unfavored to gain an additional electron.

9. What is the trend for electron gain enthalpy?

Generally, electron gain enthalpy becomes more negative as you move from left to right across a period due to increasing nuclear charge, which attracts electrons more strongly. However, there are exceptions, particularly with noble gases and specific transition metals. As you move down a group, the electron gain enthalpy usually becomes less negative due to increased atomic radius and shielding effect.

10. What is electron gain enthalpy?
Electron gain enthalpy is the energy change that occurs when an isolated gaseous atom gains an electron to form a negatively charged ion. It measures how easily an atom can accept an electron and is an important concept in understanding chemical reactivity and bonding.
11. How does electron gain enthalpy differ from ionization energy?
Electron gain enthalpy involves an atom gaining an electron, while ionization energy involves an atom losing an electron. Electron gain enthalpy measures the energy change when an atom becomes a negative ion, whereas ionization energy measures the energy required to remove an electron from an atom to form a positive ion.
12. How does electronegativity relate to electron gain enthalpy?
Electronegativity and electron gain enthalpy are closely related. Generally, elements with high electronegativity also have more negative electron gain enthalpies. Both properties measure an atom's ability to attract electrons, with electronegativity describing this tendency in a bonded context and electron gain enthalpy in an isolated atomic context.
13. How is electron gain enthalpy measured experimentally?
Electron gain enthalpy is typically measured indirectly through a combination of experimental techniques and theoretical calculations. Methods include electron impact studies, photoelectron spectroscopy, and equilibrium constant measurements of gas-phase reactions. These experimental results are often combined with quantum mechanical calculations to determine accurate values.
14. Can an atom have multiple electron gain enthalpies?
Yes, an atom can have multiple electron gain enthalpies. The first electron gain enthalpy refers to the energy change when a neutral atom gains one electron. Subsequent electron gain enthalpies (second, third, etc.) refer to the energy changes when an already negatively charged ion gains additional electrons. These subsequent values are usually positive and larger in magnitude.
15. How does electron gain enthalpy affect chemical bonding?
Electron gain enthalpy plays a crucial role in chemical bonding, especially in ionic bond formation. Elements with highly negative electron gain enthalpies (like halogens) are more likely to form anions and participate in ionic bonding with elements that have low ionization energies. It also influences the polarity of covalent bonds and the reactivity of atoms in various chemical processes.
16. Why do some sources report electron gain enthalpies in kJ/mol while others use eV?
The units kJ/mol and eV (electron volts) are both commonly used for reporting electron gain enthalpies. kJ/mol is typically used in chemistry contexts as it relates to molar quantities, while eV is often used in physics and spectroscopy as it relates to individual atoms or molecules. The choice of units depends on the context and audience, but they can be easily converted (1 eV ≈ 96.49 kJ/mol).
17. Why do noble gases have positive electron gain enthalpies?
Noble gases have positive electron gain enthalpies because they already have a stable, full outer electron shell. Adding an electron to a noble gas requires energy to overcome the repulsion from the existing electrons and to place the new electron in a higher energy level. This makes the process endothermic, resulting in a positive electron gain enthalpy.
18. Why does fluorine have a lower electron gain enthalpy than chlorine?
Fluorine has a lower (less negative) electron gain enthalpy than chlorine, which seems counterintuitive. This is due to fluorine's small size, which leads to high electron-electron repulsion in its small 2p orbital. The incoming electron experiences strong repulsion from the existing electrons, making it less energetically favorable compared to the larger chlorine atom.
19. Why is electron gain enthalpy sometimes called electron affinity?
Electron gain enthalpy is often called electron affinity because it measures an atom's affinity or tendency to attract and gain an electron. The terms are used interchangeably, though electron gain enthalpy is more precise as it specifically refers to the enthalpy change involved in the process.
20. Is electron gain enthalpy always exothermic?
No, electron gain enthalpy is not always exothermic. While most atoms release energy when gaining an electron (exothermic process), some atoms, like noble gases, require energy to add an electron (endothermic process). The sign of the enthalpy change indicates whether the process is exothermic (negative) or endothermic (positive).
21. Why do halogens have the highest electron gain enthalpies?
Halogens have the highest electron gain enthalpies because they are one electron short of a stable noble gas configuration. They have high effective nuclear charge and small atomic size, which strongly attract electrons. Gaining an electron allows them to achieve a full outer shell, resulting in a significant release of energy.
22. How does atomic size affect electron gain enthalpy?
Atomic size generally has an inverse relationship with electron gain enthalpy. As atomic size decreases, electron gain enthalpy becomes more negative (more energy is released). This is because smaller atoms have a stronger attraction between the nucleus and the incoming electron, making it easier to add an electron.
23. What is the trend of electron gain enthalpy across a period in the periodic table?
Across a period in the periodic table, electron gain enthalpy generally becomes more negative (more energy is released) from left to right. This trend is due to increasing effective nuclear charge and decreasing atomic size, which make it easier for atoms to attract and gain electrons as you move across the period.
24. Why is the second electron gain enthalpy usually positive?
The second electron gain enthalpy is usually positive because it involves adding an electron to an already negatively charged ion. The electrostatic repulsion between the incoming electron and the existing negative charge makes it energetically unfavorable to add another electron, resulting in an endothermic process with a positive enthalpy change.
25. How does the concept of electron gain enthalpy apply to molecules?
While electron gain enthalpy is primarily defined for isolated atoms, the concept can be extended to molecules. Molecular electron gain enthalpy refers to the energy change when a molecule gains an electron to form a molecular anion. This property is important in understanding the behavior of molecular species in various chemical and biochemical processes.
26. How does temperature affect electron gain enthalpy?
Temperature generally has a minimal direct effect on electron gain enthalpy values, as they are typically measured and reported for isolated atoms in the gas phase at standard conditions. However, temperature can indirectly affect electron gain processes in chemical reactions by influencing the kinetics and thermodynamics of the overall reaction system.
27. How does the octet rule relate to electron gain enthalpy trends?
The octet rule is closely related to electron gain enthalpy trends. Elements that can achieve a full octet (8 valence electrons) by gaining electrons tend to have more negative electron gain enthalpies. This explains why halogens, which are one electron short of a full octet, have the most negative electron gain enthalpies, while noble gases, which already have full octets, have positive values.
28. How does electron gain enthalpy differ for atoms and ions of the same element?
Electron gain enthalpy differs significantly for atoms and ions of the same element. The first electron gain enthalpy (for a neutral atom) is typically negative for many elements. However, the second electron gain enthalpy (for the resulting anion) is usually positive and much larger in magnitude. This is due to the increased electrostatic repulsion when adding an electron to an already negatively charged ion.
29. How does the concept of electron gain enthalpy apply in organic chemistry?
In organic chemistry, electron gain enthalpy is relevant in understanding the behavior of functional groups and their reactivity. It helps explain the electron-accepting properties of certain groups, like carbonyls or nitro groups, which can act as electron acceptors in reactions. The concept is also useful in predicting the stability of organic anions and the direction of electron flow in organic reactions.
30. How does relativistic effects influence electron gain enthalpies of heavy elements?
Relativistic effects significantly influence the electron gain enthalpies of heavy elements, particularly those in the lower right of the periodic table. These effects can cause contraction of s and p orbitals and expansion of d and f orbitals, altering electronic properties. For instance, the unexpectedly high electron gain enthalpy of gold compared to silver is partly attributed to relativistic effects.
31. How does electron gain enthalpy change down a group in the periodic table?
Electron gain enthalpy generally becomes less negative (less energy is released) as you move down a group in the periodic table. This is because atomic size increases down a group, resulting in a weaker attraction between the nucleus and the incoming electron. The effect of increased shielding by inner electrons also contributes to this trend.
32. How does electron configuration influence electron gain enthalpy?
Electron configuration significantly influences electron gain enthalpy. Atoms with nearly full valence shells (like halogens) have more negative electron gain enthalpies because gaining an electron completes their octet. Atoms with half-filled or fully filled subshells (like nitrogen or noble gases) have less negative or even positive electron gain enthalpies due to the stability of their electron configurations.
33. Can electron gain enthalpy values be used to predict chemical reactivity?
Yes, electron gain enthalpy values can be used to predict chemical reactivity to some extent. Elements with highly negative electron gain enthalpies are more likely to act as oxidizing agents and accept electrons in chemical reactions. However, reactivity predictions should also consider other factors like ionization energy, electronegativity, and specific reaction conditions.
34. Why is the electron gain enthalpy of beryllium positive?
Beryllium has a positive electron gain enthalpy because it has a stable electron configuration with a full 2s orbital. Adding an electron would require placing it in the higher energy 2p orbital, which is energetically unfavorable. Additionally, the small size of beryllium leads to high electron-electron repulsion, further contributing to the positive electron gain enthalpy.
35. What role does electron gain enthalpy play in the formation of ionic compounds?
Electron gain enthalpy plays a crucial role in the formation of ionic compounds. It represents the energy released when a non-metal accepts an electron to form an anion. This energy, combined with the ionization energy of a metal, contributes to the overall lattice energy of the ionic compound. More negative electron gain enthalpies favor the formation of stable ionic compounds.
36. How does electron gain enthalpy compare for metals and non-metals?
Non-metals generally have more negative electron gain enthalpies compared to metals. This is because non-metals have a higher tendency to gain electrons to achieve stable electron configurations. Metals, on the other hand, typically have less negative or even positive electron gain enthalpies, as they tend to lose electrons rather than gain them in chemical reactions.
37. Can electron gain enthalpy be zero?
While it's theoretically possible for an electron gain enthalpy to be exactly zero, it's extremely rare and unlikely to occur in practice. Most elements have non-zero electron gain enthalpies, either positive or negative. Values close to zero may be observed for some elements, but precise zero values are not typically reported due to experimental uncertainties and the nature of atomic interactions.
38. How does the concept of shielding affect electron gain enthalpy?
Shielding, or electron shielding, significantly affects electron gain enthalpy. Inner electrons shield outer electrons from the full attractive force of the nucleus. As you move down a group in the periodic table, the increased shielding from additional electron shells reduces the attraction between the nucleus and the incoming electron. This results in less negative electron gain enthalpies as you descend a group.
39. Why is the electron gain enthalpy of chlorine more negative than that of sulfur?
Chlorine has a more negative electron gain enthalpy than sulfur because it requires only one electron to achieve a stable octet configuration, while sulfur requires two. Chlorine's higher effective nuclear charge and smaller atomic size also contribute to its stronger attraction for an additional electron. Sulfur's less negative value reflects the energy cost of overcoming electron-electron repulsion when adding an electron to its partially filled 3p orbital.
40. How does electron gain enthalpy relate to the reactivity of halogens?
Electron gain enthalpy is closely related to the reactivity of halogens. The highly negative electron gain enthalpies of halogens contribute to their strong oxidizing properties and high reactivity. Fluorine, with the most negative electron gain enthalpy among halogens, is the most reactive. The trend in reactivity (F > Cl > Br > I) generally follows the trend in electron gain enthalpy, although other factors also play a role.
41. What is the significance of electron gain enthalpy in understanding redox reactions?
Electron gain enthalpy is significant in understanding redox (reduction-oxidation) reactions. It provides insight into an element's tendency to be reduced (gain electrons) in a reaction. Species with more negative electron gain enthalpies are stronger oxidizing agents, as they more readily accept electrons. This property helps predict the direction of electron transfer in redox reactions and the relative strengths of oxidizing agents.
42. Why isn't electron gain enthalpy simply the negative of ionization energy?
Electron gain enthalpy is not simply the negative of ionization energy because these processes involve different electronic transitions and energy states. Ionization energy involves removing an electron from a neutral atom, while electron gain enthalpy involves adding an electron to a neutral atom. The energy changes in these processes are influenced by different factors, including electron-electron repulsion and changes in electronic configuration.
43. How does spin pairing affect electron gain enthalpy?
Spin pairing can significantly affect electron gain enthalpy. When an incoming electron must pair with an existing electron in an orbital, it requires additional energy to overcome the electron-electron repulsion. This is why elements with half-filled subshells (like nitrogen with its half-filled p orbital) often have less negative electron gain enthalpies than might be expected based on other periodic trends.
44. Can electron gain enthalpy values be used to predict the stability of anions?
Yes, electron gain enthalpy values can be used to predict the stability of anions to some extent. More negative electron gain enthalpies generally indicate more stable anions, as more energy is released when forming these ions. However, the stability of anions in real chemical systems also depends on other factors, such as the surrounding environment, solvent effects, and lattice energies in solid compounds.
45. Why do some transition metals have low electron gain enthalpies despite being metals?
Some transition metals have relatively low (more negative) electron gain enthalpies compared to other metals due to their partially filled d-orbitals. Adding an electron to these d-orbitals can sometimes lead to a more stable electronic configuration. However, their electron gain enthalpies are still generally less negative than those of non-metals, reflecting the overall metallic character of these elements.
46. How does electron gain enthalpy relate to electrochemical potential?
Electron gain enthalpy is related to electrochemical potential, particularly the standard reduction potential. Elements with more negative electron gain enthalpies tend to have more positive standard reduction potentials, indicating a greater tendency to be reduced (gain electrons) in electrochemical reactions. This relationship helps in understanding and predicting the behavior of elements in electrochemical processes.
47. Can electron gain enthalpy be used to explain the formation of certain compounds?
Yes, electron gain enthalpy can be used to explain the formation of certain compounds, especially ionic compounds. For example, the highly negative electron gain enthalpy of chlorine and the low ionization energy of sodium help explain the formation of sodium chloride. The energy released when chlorine gains an electron contributes to the overall stability of the ionic compound.
48. How does the concept of electron gain enthalpy apply to semiconductor materials?
In semiconductor materials, electron gain enthalpy is relevant to understanding the behavior of electrons and holes. It relates to the energy required to move electrons from the valence band to the conduction band. While not directly equivalent, the concept helps in comprehending electron affinity levels in semiconductors, which are crucial for designing and understanding electronic devices.
49. Why is there no clear trend in electron gain enthalpies for transition metals?
There is no clear trend in electron gain enthalpies for transition metals due to their complex electronic structures. The presence of partially filled d-orbitals leads to various possible electron configurations when an electron is added. This variability, combined with the interplay between electron-electron repulsion and nuclear attraction, results in irregular patterns of electron gain enthalpies across the transition metal series.
50. Can electron gain enthalpy be negative for cations?
Yes, electron gain enthalpy can be negative for cations. When a cation gains an electron, it becomes a neutral atom or a less positively charged ion. This process often releases energy, resulting in a negative electron gain enthalpy. For example, the electron gain enthalpy for a proton (H+) to form a hydrogen atom is highly

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