Electron Affinity vs Electron Gain Enthalpy: Sign Convention (NEET)
Chemistry · Periodic Classification Of Properties · NEET
When an atom takes an extra electron and releases energy, electron gain enthalpy (Δ_egH) is NEGATIVE, but the old term electron affinity (A) is POSITIVE. They describe the same event; only the sign flips because Δ_egH = -A (roughly). Memory hook: "Gain enthalpy Goes negative when energy is Given out."
For the same electron-gain process, electron gain enthalpy is negative (energy released, thermodynamic sign) while the older electron affinity is written positive; the two are linked by Δ_egH ≈ -A.
Your doubts, answered
Is electron gain enthalpy positive or negative when energy is released?
It is NEGATIVE. In thermodynamics, energy released by a system is negative. When a neutral atom adds an electron and lets out energy (like Cl → Cl⁻), Δ_egH is a negative number, for example about -349 kJ/mol for chlorine. Negative sign = energy leaves the atom.
So why do books say electron affinity is positive for the same reaction?
Electron affinity (A) is an OLDER term. It is defined as the energy RELEASED, so it is written as a positive amount when energy comes out. Electron gain enthalpy (Δ_egH) is the modern thermodynamic term and uses the opposite sign for release. That is why Δ_egH ≈ -A. Same physical event, opposite signs.
What is the exact relation between electron affinity and electron gain enthalpy?
At absolute zero, Δ_egH = -A. At normal temperature there is a tiny correction: Δ_egH = -A - (5/2)RT, coming from the (5/2)RT change when a free electron is absorbed. For NEET, the key point you must remember is the SIGN flip: release of energy → A positive, Δ_egH negative.
When is electron gain enthalpy POSITIVE (endothermic)?
When you must FORCE an electron onto an atom that does not want it. Noble gases (stable full shells) and adding a second electron to an already negative ion (like O⁻ → O²⁻) need energy input, so Δ_egH is positive. Here electron affinity is treated as negative, which again shows the sign flip.
Which term does NEET use in questions and answers?
NEET follows NCERT, which uses electron gain enthalpy (Δ_egH) with the modern sign convention. 'More negative Δ_egH' means the atom accepts an electron more easily. If a question says 'high electron affinity', it means the same as 'very negative electron gain enthalpy'. Read the wording carefully.
Does 'most negative electron gain enthalpy' mean strongest or weakest attraction?
Most negative = STRONGEST attraction for the extra electron. Chlorine has the most negative Δ_egH of all elements, so it accepts an electron most readily. A less negative (or positive) value means the atom holds the incoming electron weakly or rejects it.
⚠️ The NEET trap ✗ Fluorine is the smallest halogen, so it must have the most negative electron gain enthalpy: F > Cl > Br > I. ✓ Chlorine, not fluorine, has the most negative electron gain enthalpy. Order is Cl > F > Br > I. Fluorine's 2p subshell is very small, so the extra electron feels strong repulsion, making its Δ_egH less negative than chlorine's. 🧠 Smallest is NOT always most negative — F is too tiny, its electrons crowd and repel, so Cl wins.
Real NEET questions
ReNEET 2026
Among the following, the correct trend in electron gain enthalpy (most negative first) is:
A · F > Cl > Br > I
B · Br > Cl > F > I
C · Cl > F > Br > I ✓
D · I > Br > Cl > F
Solution: Down group 17, electron gain enthalpy usually becomes less negative. But fluorine is anomalous: its very small 2p subshell causes strong electron-electron repulsion when the extra electron enters, so its Δ_egH is less negative than chlorine's. The correct order of magnitude (most negative first) is Cl > F > Br > I. This directly uses the sign convention: 'most negative' = accepts electron most easily.
NEET 2016 Phase 1
In which option does the order NOT agree with the property indicated? (relevant option: I < Br < Cl < F, increasing electron gain enthalpy)
B · B < C < N < O (increasing first ionization enthalpy) ✓
C · I < Br < Cl < F (increasing electron gain enthalpy)
D · Li < Na < K < Rb (increasing metallic radius)
Solution: The official answer is B (nitrogen's stable half-filled 2p³ makes its first ionization enthalpy higher than oxygen's). Note that option C treats 'increasing electron gain enthalpy' by MAGNITUDE going I < Br < Cl < F; strictly Cl is more negative than F, so the halogen anomaly is a known subtlety NEET tests. The key learning: signs and magnitudes of Δ_egH must be read carefully.
Solved Periodic Classification Of Properties NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Are electron affinity and electron gain enthalpy the same thing?
They describe the same process — a neutral atom gaining one electron — but electron affinity is the older term (energy released, taken positive) and electron gain enthalpy is the modern thermodynamic term (release taken negative). So Δ_egH ≈ -A.
Why is the electron gain enthalpy of chlorine negative?
Because chlorine releases energy when it accepts an electron to become Cl⁻. Energy leaving the system is written with a negative sign, so Δ_egH(Cl) ≈ -349 kJ/mol, the most negative of all elements.
Which term should I use in the NEET exam?
Use electron gain enthalpy with the modern sign convention, because NCERT and NEET follow it. If a question says 'electron affinity', mentally flip the sign to match Δ_egH.
Is a positive electron gain enthalpy possible?
Yes. Noble gases and second-electron additions (O⁻ → O²⁻) have positive Δ_egH because energy must be supplied to force the electron in.
Does 'more negative' Δ_egH mean the atom is more metallic or non-metallic?
More non-metallic. Non-metals like halogens strongly attract electrons and have very negative Δ_egH. Metals attract added electrons weakly, so their Δ_egH is small or positive.
