Electron Affinity vs Electronegativity: Key Differences & Table
Electron Affinity is essential in chemistry and helps students understand various practical and theoretical applications related to atom and ion formation. Knowing about electron affinity allows students to predict which elements will gain electrons easily and form negative ions, an important step in understanding periodic table trends and reactivity.
What is Electron Affinity in Chemistry?
Electron affinity refers to the energy change that occurs when an atom in the gaseous state gains an extra electron to form a negative ion. This important concept appears in chapters related to atomic structure, periodic table trends, and chemical bonding, making it a foundational part of your chemistry syllabus. A higher (more negative) electron affinity means the atom attracts electrons more strongly.
Molecular Formula and Composition
Unlike molecules with fixed formulas, electron affinity is a property rather than a specific compound or molecule. It applies to individual atoms, especially nonmetals like chlorine (Cl) or oxygen (O), and is measured in kilojoules per mole (kJ/mol). The general process can be written as: X(g) + e⁻ → X⁻(g).
Preparation and Synthesis Methods
You cannot “prepare” electron affinity itself, but you can calculate or measure it in laboratories using advanced instruments. Methods like spectroscopic techniques, the Born-Haber cycle, and thermochemical data are used to determine the electron affinity of different elements.
Physical Properties of Electron Affinity
Electron affinity is a numerical value usually expressed in kJ/mol. For most elements, electron affinity values are negative, meaning energy is released when an electron is added. For example, chlorine’s electron affinity is about -349 kJ/mol, which is the highest among all elements. Noble gases have values close to zero or even positive, meaning they do not attract extra electrons easily.
Chemical Properties and Reactions
Although electron affinity is not a chemical itself, it predicts how easily an element forms negative ions through reactions. For instance, elements with high electron affinity form stable negative ions (anions) like Cl⁻ or O²⁻, which then participate in ionic bonds with metals. Second electron affinity, such as when O⁻(g) gains another electron to form O²⁻(g), requires energy (positive electron affinity) due to electron repulsion.
Frequent Related Errors
- Confusing electron affinity with electronegativity or electron gain enthalpy.
- Assuming all nonmetals have the same electron affinity values.
- Ignoring periodic trends and group exceptions (like fluorine and noble gases).
- Thinking the process always releases energy (second electron affinity is endothermic).
Uses of Electron Affinity in Real Life
Electron affinity helps predict reactivity and bonding in industrial processes, such as manufacturing bleach, semiconductors, and fertilizers. It also guides chemists in designing chemical reactions for electronics, water treatment, and pharmaceuticals. Elements with high electron affinity are used to make strong oxidizing agents.
Relevance in Competitive Exams
Students preparing for NEET, JEE, and Olympiads should be familiar with electron affinity, as it often features in concept and calculation questions regarding the periodic table, atomic trends, and chemical bonding. Knowing electron affinity trends is also useful for CBSE board exams, especially when comparing it to electronegativity or ionization energy.
Relation with Other Chemistry Concepts
Electron affinity is closely related to topics such as electronegativity, electron gain enthalpy, and periodic table trends. Understanding it helps students build links between atomic structure, the nature of chemical bonds, and element reactivity.
Step-by-Step Reaction Example
1. Write the electron affinity addition process for chlorine:Cl(g) + e⁻ → Cl⁻(g) ΔE = -349 kJ/mol
2. State what happens:
The chlorine atom in the gaseous state attracts an extra electron, releasing energy and forming a stable Cl⁻ ion.
3. Explain second electron affinity using oxygen:
O(g) + e⁻ → O⁻(g) ΔE₁ = -141 kJ/mol (energy released)
O⁻(g) + e⁻ → O²⁻(g) ΔE₂ = +780 kJ/mol (energy absorbed)
4. Final Answer: First electron affinity is exothermic; second is endothermic due to repulsion.
Lab or Experimental Tips
Remember electron affinity by linking it to “energy released when an electron is gained.” Vedantu educators often advise drawing quick tables of values or visualizing group/period trends with arrows in live classes for fast revision. Use “up and right on the periodic table means higher electron affinity, except for noble gases.”
Try This Yourself
- Compare the electron affinity of fluorine and chlorine—who has the higher value and why?
- List three elements with very low electron affinity values.
- Is the electron affinity of noble gases positive, zero, or negative? Explain why.
- Draw a simple periodic table trend arrow map for electron affinity.
Final Wrap-Up
We explored electron affinity—its meaning, process, and why it matters for reactions and the periodic table. You now understand how electron affinity connects to ion formation, exceptions across the periodic table, and related chemistry topics. For more exam-focused notes, solved problems, and live guidance on this and other chemistry concepts, check out Vedantu’s other topic pages and join a live class!
FAQs on Electron Affinity Explained: Trends, Definition & Examples
1. What is electron affinity in chemistry?
Electron affinity is the energy change that occurs when a neutral atom in its gaseous state accepts an electron to form a negatively charged ion (anion). A more negative value indicates a greater affinity for an electron, as energy is released in an exothermic process.
2. How does electron affinity generally change across the periodic table?
In the periodic table, electron affinity generally becomes more negative (increases) when moving from left to right across a period. Conversely, it becomes less negative (decreases) when moving down a group. However, there are notable exceptions to this trend.
3. Which element has the highest electron affinity?
Chlorine (Cl) has the highest (most negative) electron affinity of all elements. Although fluorine is more electronegative, its small size causes significant electron-electron repulsion in its compact 2p subshell, making its electron affinity value less negative than that of chlorine.
4. What is the difference between electron affinity and electronegativity?
Electron affinity is a quantifiable energy measurement (in kJ/mol) for an isolated, gaseous atom gaining an electron. In contrast, electronegativity is a relative, dimensionless scale that describes an atom's ability to attract shared electrons within a chemical bond. While related, electron affinity is an absolute property of a single atom, and electronegativity is a property within a molecule.
5. What key factors influence an element's electron affinity?
The primary factors affecting electron affinity are:
- Effective Nuclear Charge: A higher charge from the nucleus attracts an incoming electron more strongly.
- Atomic Radius: A smaller atom typically has a higher electron affinity as the incoming electron is closer to the nucleus.
- Electron Configuration: Atoms with stable configurations (e.g., filled or half-filled subshells) have low electron affinities as adding an electron disrupts their stability.
- Shielding Effect: Inner-shell electrons repel the incoming electron, reducing the pull from the nucleus.
6. Why does chlorine have a more negative electron affinity than fluorine?
This anomaly is due to fluorine's extremely small atomic size. The high electron density in fluorine's compact 2p subshell leads to strong inter-electronic repulsion when a new electron is added. In chlorine, the incoming electron enters the larger 3p orbital, where there is more space and less repulsion, resulting in a more exothermic energy release and a more negative electron affinity.
7. Why does electron affinity generally increase across a period?
Moving from left to right across a period, electrons are added to the same principal energy level while the number of protons in the nucleus increases. This leads to a higher effective nuclear charge. The stronger attraction from the nucleus for valence electrons makes it more energetically favourable to add another electron, resulting in a more negative electron affinity value.
8. What is second electron affinity and why is it always a positive value?
Second electron affinity is the energy change when an electron is added to a negatively charged ion (anion). Its value is always positive because energy is required to force an electron to overcome the strong electrostatic repulsion from the already-negative ion. This makes the process endothermic, requiring an input of energy.
9. How does electron affinity's value help in understanding chemical reactivity?
Electron affinity is a crucial indicator of reactivity. Elements with a high negative electron affinity, like the halogens, readily accept electrons to form stable anions. This high tendency to gain electrons makes them strong oxidising agents and highly reactive with metals to form ionic compounds. Conversely, elements with low or positive electron affinity, like metals and noble gases, are less likely to accept electrons.
10. Why do some elements, like noble gases, have positive electron affinities?
A positive electron affinity signifies that energy must be supplied to add an electron. This occurs in elements that already possess a stable electron configuration. For example, noble gases have a completely filled valence shell. Adding an electron would place it in a new, higher-energy shell, which is an energetically unfavourable (endothermic) process, thus resulting in a positive value.
