Electron gain enthalpy (ΔegH) is the enthalpy change when an isolated gaseous atom gains one electron to form a monovalent anion:
X(g) + e⁻ → X⁻(g) ; ΔegH
The sign convention matters: a negative ΔegH means energy is released (exothermic, favourable); a positive value means energy must be supplied (endothermic, unfavourable).
General periodic trends:
Key anomalies you must know:
Fluorine vs Chlorine: F has a less negative ΔegH than Cl. Fluorine's tiny 2p orbital is already crowded — severe electron–electron repulsion destabilises the incoming electron. Chlorine's larger 3p orbital accommodates the extra electron more comfortably. So the order is Cl > F (more negative for Cl).
Noble gases and alkaline earth metals: Atoms with completely filled subshells (ns², ns²np⁶) have positive or near-zero ΔegH — they resist gaining an electron because the added electron must enter a higher-energy shell.
Nitrogen (half-filled 2p³): Has near-zero or slightly positive ΔegH because the extra stability of a half-filled p subshell makes electron addition unfavourable.
Oxygen vs Sulphur: O has a less negative ΔegH than S, paralleling the F vs Cl anomaly — small compact 2p orbitals create electron–electron repulsion.
NEET watch-out: Questions frequently present four elements and ask which has the most negative ΔegH. The distractor is F when the answer is Cl, exploiting the assumption that "smallest halogen = highest affinity."
(Reference: NCERT Class 11 Chemistry Chapter 3, page 18)
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Electron gain enthalpy is defined as the enthalpy change when:
Which of the following has the most negative electron gain enthalpy?
The electron gain enthalpy of nitrogen is approximately zero (or slightly positive). This is because:
Among O, S, Se, Te — the element with the most negative electron gain enthalpy is:
The electron gain enthalpy of Be, Mg, and Ne is positive. The common reason is:
Arrange the following in order of increasingly negative electron gain enthalpy: Na, Cl, Ar, S.
Which statement about electron gain enthalpy is INCORRECT?
The second electron gain enthalpy of oxygen (O⁻ → O²⁻) is positive because:
Pattern: Periodic trends comparison — electron gain enthalpy ordering (from NEET pattern: periodic trend bundle, which includes CHE.U09.ELECTRON_GAIN_ENTHALPY in its topic_codes).
Given
Four elements: F (2p⁵), Cl (3p⁵), O (2p⁴), S (3p⁴).
Required
Order from least negative to most negative ΔegH.
Concept
- Halogens (Group 17) have more negative ΔegH than chalcogens (Group 16) in the same period — one electron short of octet vs two. - Within a group, the second-period element (F, O) has less negative ΔegH than the third-period element (Cl, S) due to compact 2p orbital electron–electron repulsion.
Framework (no formula needed — trend-based reasoning)
- Compare within groups first: Cl > F (more negative) and S > O (more negative). - Compare across period: Cl > S (halogen vs chalcogen in period 3); F > O in period 2, but F may be close to S. - Known values: Cl (−349) > F (−328) > S (−200) > O (−141) kJ/mol.
Substitution
Applying the two principles: - O is least negative (compact 2p⁴ + only Group 16). - S is next (larger 3p⁴, less repulsion than O, but still Group 16). - F is next (compact 2p⁵ — halogen but repulsion penalty). - Cl is most negative (3p⁵ — halogen with spacious orbital).
Ordering
Least negative → most negative: O < S < F < Cl
Final answer
O (−141 kJ/mol) < S (−200 kJ/mol) < F (−328 kJ/mol) < Cl (−349 kJ/mol)
Common trap
Placing F as most negative by assuming "top of group = highest affinity." The F vs Cl anomaly (inter-electron repulsion in the compact 2p shell) is a high-frequency NEET distractor. Similarly, placing O above S ignores the same 2p repulsion effect.
Similar NEET-style question
"Which of the following represents the correct order of electron gain enthalpy (most negative first)? (a) Cl > F > S > O (b) F > Cl > O > S (c) Cl > F > O > S (d) F > Cl > S > O" — Answer: (a).
Energy released when neutral atom gains electron. Across period: more negative (favourable). Down group: becomes less negative. Cl > F (Cl has larger size, less e⁻-e⁻ repulsion).
-- NCERT Class 11 Chemistry, Ch. 3, p. 18Energy required to ionize an electron from the n-th shell of hydrogen-like atom.
| Symbol | Quantity | SI Unit |
|---|---|---|
| Z | nuclear charge | - |
| n | quantum number | - |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Student includes inert-gas radius in atomic-radius trends. But inert gases use van der Waals radius (much larger than covalent), making 'monotonic decrease across period' look broken.
Atomic radius comparison includes a noble gas or trends across period 2/3.
Compare like with like: covalent radii for non-noble gases. Noble gas radii are van der Waals (no covalent bond). Don't compare noble-gas radius directly to halogen.
Category: Inorganic Exception
Student expects monotonic increase in IE across period. Anomalies: Be(s²) > B(s²p¹); N(p³ half-filled) > O(p⁴).
Compare IE values across period 2 (Li, Be, B, C, N, O, F).
Be > B (s² stable; B's p¹ easier to remove). N > O (N has p³ half-filled stability; O loses one to attain p³). Memorise these two anomalies.
Root cause: concept gap
Be>B (s² stability); N>O (N's p³ half-filled stability). Memorise these two anomalies in period 2.
Root cause: concept gap
Don't compare different radius types. Noble gases use vdW radius (much larger); halogens use covalent radius. Compare like-with-like.
3 questions from NEET 2021, 2024. Answers verified against NTA official keys.
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
swapped classes
Tempts surface-level recall.
Test yourself on this topic with real past-paper questions:
Practice this topic →Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.