JEE Mains Chemistry · Classification of Elements and Periodicity
Electron Gain Enthalpy
Electron gain enthalpy is the enthalpy change when a gaseous atom takes an electron; it is negative for most atoms, positive for noble gases, and Cl, not F, has the most negative value.
Why this matters
Sixteen PYQs, all multiple choice, and three from 2026. Six turn on the sign: which atoms release energy on gaining an electron and how the noble gases rank. Ten rank groups 16 and 17, where F and O break the trend, and the options often differ only in whether the order is read on signed values or on magnitudes.
Concept 1 of 2: When electron gain releases energy and when it costs energy
An atom that is one or two electrons short of a noble-gas shell welcomes an extra electron and gives out energy, so its electron gain enthalpy is negative. A noble gas would have to put the electron into a new, higher shell, so energy must be supplied and the value is positive. An anion repels a second electron, so O⁻ → O²⁻ also costs energy.
Definition
- negative = exothermic; positive = endothermic.
- Positive for all noble gases, and for Be (filled ), N (half-filled ) and the second electron added to .
- Negative for the alkali metals (Na −53) and for Al, though small.
- Across a period becomes more negative; down a group it becomes less negative.
- Electron affinity is quoted as energy released, so its sign is the opposite: a negative electron affinity means the process is endothermic.
| Atom | Electron gain enthalpy (kJ mol⁻¹) | Sign | Why |
|---|---|---|---|
| He | +48 | Endothermic | Electron must enter the shell |
| Ne | +116 | Endothermic | Most positive noble gas; the electron enters |
| Ar | +96 | Endothermic | Same value as Kr |
| Kr | +96 | Endothermic | Same value as Ar |
| Xe | +77 | Endothermic | Larger atom, smaller cost |
| Li | −60 | Exothermic | Half-filled takes a second s electron |
| Na | −53 | Exothermic | Small but negative |
| Cl | −349 | Exothermic | Most negative of all elements |
The largest gap between two elements is Ne and Cl: kJ mol⁻¹.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2023 · 25 Jan 2023 · Q44Moderate
Example 1 · Classification of Elements and Periodicity · Electron Gain Enthalpy
Inert gases have positive electron gain enthalpy. Its correct order is
Neon, not helium, is the most positive
Helium's value (+48) is the smallest of the noble gases, and neon's (+116) the largest. Ar and Kr are equal at +96, which is why options that separate them need care.
Electron affinity flips the sign
A question that says the electron affinity is negative for Be, N and O → O²⁻ means those processes absorb energy. Electron affinity counts energy released, so it has the opposite sign to .
Concept 2 of 2: Groups 16 and 17: why Cl beats F and S beats O
F and O are so small that their 2p shell is crowded. An incoming electron is repelled by the electrons already packed in, so less energy is released than for the next element down. That is why Cl has the most negative value of all, and O has the least negative value in its group.
Definition
- Group 17 by magnitude: Cl > F > Br > I > At.
- Group 16 by magnitude: S > Se > Te > Po > O.
- Across a period the halogen is the most negative element; the alkali metal has the lowest ionization enthalpy.
- On signed values, "less than" means more negative: is true.
- Some papers write the order by magnitude (Cl > F). Before choosing, check which reading the options use.
| Group | Electron gain enthalpy (kJ mol⁻¹) | Order by magnitude |
|---|---|---|
| 17 | F −328, Cl −349, Br −325, I −295, At −270 | Cl > F > Br > I > At F is second, not first. Cl is the most negative element in the table. |
| 16 | O −141, S −200, Se −195, Te −190, Po −174 | S > Se > Te > Po > O O is the least negative in group 16, below even Po. |
| 1 | Li −60, Na −53, K −48, Rb −47, Cs −46 | Li > Na > K > Rb ≈ Cs |
| Hydrogen | H −73 | More negative than any alkali metal |
Down each group the value becomes less negative, except that the first member of groups 16 and 17 is out of place.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2023 · 1 February 2023 · Q121Moderate
Example 2 · Classification of Elements and Periodicity · Electron Gain Enthalpy
For electron gain enthalpies of the elements denoted as , the incorrect option is:
F is not the most negative
Fluorine has the highest electronegativity, but not the most negative electron gain enthalpy. Its small shell repels the added electron. Any statement that F's value is more negative than Cl's is false.
Signed or magnitude
"S > Se > Te > O" is right by magnitude and wrong on signed values. When one option is the exact reverse of another, the paper is testing which reading it means; pick the one consistent with the other statements in the question.
Summary — formulas & gotchas at a glance
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Reference tables (2)
When electron gain releases energy and when it costs energy8 rows
| Atom | Electron gain enthalpy (kJ mol⁻¹) | Sign | Why |
|---|---|---|---|
| He | +48 | Endothermic | Electron must enter the shell |
| Ne | +116 | Endothermic | Most positive noble gas; the electron enters |
| Ar | +96 | Endothermic | Same value as Kr |
| Kr | +96 | Endothermic | Same value as Ar |
| Xe | +77 | Endothermic | Larger atom, smaller cost |
| Li | −60 | Exothermic | Half-filled takes a second s electron |
| Na | −53 | Exothermic | Small but negative |
| Cl | −349 | Exothermic | Most negative of all elements |
The largest gap between two elements is Ne and Cl: kJ mol⁻¹.
Groups 16 and 17: why Cl beats F and S beats O4 rows
| Group | Electron gain enthalpy (kJ mol⁻¹) | Order by magnitude |
|---|---|---|
| 17 | F −328, Cl −349, Br −325, I −295, At −270 | Cl > F > Br > I > At F is second, not first. Cl is the most negative element in the table. |
| 16 | O −141, S −200, Se −195, Te −190, Po −174 | S > Se > Te > Po > O O is the least negative in group 16, below even Po. |
| 1 | Li −60, Na −53, K −48, Rb −47, Cs −46 | Li > Na > K > Rb ≈ Cs |
| Hydrogen | H −73 | More negative than any alkali metal |
Down each group the value becomes less negative, except that the first member of groups 16 and 17 is out of place.
Watch out for (4)
- Neon, not helium, is the most positive→ When electron gain releases energy and when it costs energy
- Electron affinity flips the sign→ When electron gain releases energy and when it costs energy
- F is not the most negative→ Groups 16 and 17: why Cl beats F and S beats O
- Signed or magnitude→ Groups 16 and 17: why Cl beats F and S beats O
Test yourself on Classification of Elements and Periodicity
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