Anand Classes brings you a clear and detailed explanation of Electron Gain Enthalpy (Electron Affinity), an important concept in Chemistry for JEE, NEET, and CBSE Class 11 students. Electron gain enthalpy is defined as the energy change that occurs when an electron is added to an isolated gaseous atom to form a negatively charged ion. Understanding this concept helps students grasp the reactivity trends of elements, periodic variations, and the stability of ions, making it a crucial topic for competitive exams.
Table of Contents
Electron Gain Enthalpy
Just as energy is required to remove an electron from an atom, energy is released when an electron is added to a neutral atom. This is called electron gain enthalpy (or electron affinity).
Thus, electron gain enthalpy is the energy released when electron is added to an isolated gaseous atom to form the gaseous negative ion.
Definition:
Electron gain enthalpy is the energy change that occurs when an electron is added to an isolated gaseous atom to form a gaseous negative ion. $$A(g) + e^{-}(g) \;\longrightarrow\; A^{-}(g) \quad \Delta H = \Delta_{\text{eg}}H$$
It is represented as ΔegH and expressed in kilojoules per mole (kJ mol⁻¹).
- In general, electron gain enthalpy is regarded as the enthalpy change when an electron is added to an isolated gaseous atom.
- The process may be exothermic (energy released) or endothermic (energy absorbed).
Exothermic Case:
For many elements, the addition of an electron releases energy, making ΔegH negative.
- Example: Group 17 elements (halogens) have very high negative values of electron gain enthalpy because they have a strong tendency to accept an electron and achieve stable noble gas configurations.
Endothermic Case:
Some elements, such as noble gases, have large positive electron gain enthalpy values.
- Reason: They already possess stable electronic configurations and have no tendency to accept additional electrons.
- If forced to gain an electron, it must enter a higher principal quantum shell, resulting in an unstable arrangement.
Magnitude of Electron Gain Enthalpy:
- A highly negative value indicates that the atom has a strong tendency to accept an electron, releasing a large amount of energy.
- A small negative (or positive) value indicates a weaker tendency to accept an electron, releasing less energy (or requiring energy input).
FAQs on Electron Gain Enthalpy (Examination Asked Questions)
Q1. What is electron gain enthalpy?
Answer:
Electron gain enthalpy is the amount of energy released (or absorbed) when an electron is added to an isolated gaseous atom to form a negatively charged ion (anion). $$A(g) + e^{-}(g) \;\longrightarrow\; A^{-}(g) \quad \Delta H = \Delta_{\text{eg}}H$$
- If energy is released → electron gain enthalpy is negative.
- If energy is absorbed → electron gain enthalpy is positive.
Q2. Why is electron gain enthalpy usually negative?
Answer:
Most atoms have a tendency to gain extra electrons to achieve stable electronic configurations (like noble gases). Since attraction between the nucleus and the added electron releases energy, electron gain enthalpy is generally negative.
Q3. Which element has the most negative electron gain enthalpy?
Answer:
- Chlorine (Cl) has the most negative electron gain enthalpy (more than Fluorine).
- This is because fluorine atoms are small and have high electron-electron repulsion in their compact 2p subshell, while chlorine (larger in size) allows the incoming electron more easily.
Q4. What is the difference between electron gain enthalpy and ionization enthalpy?
Answer:
- Ionization enthalpy: Energy required to remove an electron from an atom.
- Electron gain enthalpy: Energy released (or absorbed) when an atom gains an electron.
Thus, ionization enthalpy is always positive, while electron gain enthalpy can be negative or positive.
Q5. Why do noble gases have positive electron gain enthalpy?
Answer:
Noble gases already have stable, completely filled electronic configurations. Adding an extra electron would require placing it in a higher-energy orbital, which is energetically unfavorable. Therefore, their electron gain enthalpy values are positive.
Q6. Why is the electron gain enthalpy of oxygen less negative than that of sulfur?
Answer:
- Oxygen is smaller in size, so when an extra electron is added, strong electron-electron repulsions occur in its compact 2p orbital.
- Sulfur, being larger, experiences lesser repulsion and can accommodate the extra electron more easily.
Thus, ΔegH of sulfur is more negative than that of oxygen.
Q7. How does electron gain enthalpy vary across a period in the periodic table?
Answer:
- From left to right across a period, electron gain enthalpy generally becomes more negative because nuclear charge increases and atomic size decreases.
- Exception: Noble gases and fluorine (due to repulsions).
Q8. How does electron gain enthalpy vary down a group in the periodic table?
Answer:
- From top to bottom in a group, electron gain enthalpy becomes less negative because atomic size increases and nuclear attraction decreases.
- Exception: Oxygen vs. sulfur, fluorine vs. chlorine.
Q9. What is successive electron gain enthalpy? Why is the second electron gain enthalpy positive?
Answer:
- First electron gain enthalpy: Energy released when the first electron is added to a neutral atom.
- Second electron gain enthalpy: Energy change when an electron is added to a negatively charged ion.
Since a negative ion strongly repels an incoming electron, energy must be supplied. Hence, the second electron gain enthalpy is always positive.
Example: $$O_{(g)} + e^- \rightarrow O^-_{(g)} \quad (\Delta H < 0)$$
$$O^-_{(g)} + e^- \rightarrow O^{2-}_{(g)} \quad (\Delta H > 0)$$
Q10. How is electron affinity different from electron gain enthalpy?
Answer:
- Electron Affinity (EA): It is the energy released when an electron is added to an atom, expressed as a positive quantity.
- Electron Gain Enthalpy (ΔegH): It is the actual energy change, which may be negative or positive.
Example: If ΔegH = –349 kJ/mol, then EA = +349 kJ/mol.
⚡ Do You Know?
- Non-metals generally have more negative electron gain enthalpy than metals.
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