Chemistry · Periodic Classification Of Properties · NEET
Normally, going down a group, electron gain enthalpy becomes less negative because the atom gets bigger and the nucleus pulls the new electron less. So you would expect F (top) to be the most negative. But F breaks this rule. F is extremely small, so its 2p shell is very compact and already crowded with electrons. When one more electron is added, it faces strong repulsion in that tiny space. Cl is bigger, its 3p shell is roomier, so the added electron settles more easily and releases more energy. Result: Cl (-349 kJ/mol) is MORE negative than F (-328 kJ/mol).
Sulphur is more negative. O has electron gain enthalpy about -141 kJ/mol, while S is about -200 kJ/mol. Even though O is higher up (and 'should' hold an electron more tightly), its small 2p shell crowds the incoming electron and repels it. S has a larger 3p shell with more room, so it accepts the electron more happily and releases more energy. This is the same reason F is less negative than Cl.
The added electron goes into the 2p subshell. In O and F, the 2p subshell is small and already holds several electrons packed close together. Electrons all carry negative charge and push each other away. In such a small region, this electron-electron repulsion is very strong. It partly cancels the pull from the nucleus, so less net energy is released. In S and Cl the electron enters the larger 3p subshell where electrons are more spread out, so repulsion is weaker.
Chlorine. NCERT states clearly: chlorine has the most negative electron gain enthalpy of all elements. This is because Cl has a strong nuclear pull AND a roomy 3p shell, so it adds an electron very easily. Fluorine, despite being more electronegative, is not the most negative because of the small-shell repulsion. Watch out: NEET often tries to trick you into picking F as 'most negative' — the correct answer is Cl.
No. Electron gain enthalpy and electronegativity are different things. Fluorine is still the MOST electronegative element (Pauling value 4.0) and is very reactive. Electron gain enthalpy is the energy released when one gas-phase atom gains one electron; electronegativity is the tendency of a bonded atom to pull shared electrons. F wins on electronegativity but loses to Cl on electron gain enthalpy because of its cramped 2p shell. Do not mix the two up.
It is a general trend for period-2 versus period-3 elements. The first member of a p-block group (N, O, F) is unusually small, so its electron gain enthalpy is less negative than the second member (P, S, Cl). After the first two members, the normal 'less negative down the group' trend takes over (Cl > Br > I). So the anomaly is specifically the 2nd-period element being smaller/less negative than the 3rd-period one right below it.
Among the following, the correct trend in electron gain enthalpy (most negative first) is:
In which option does the order NOT agree with the property indicated? (choose the wrong one) — one option reads: I < Br < Cl < F (increasing electron gain enthalpy).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Approximate values: O about -141 kJ/mol, S about -200 kJ/mol, F about -328 kJ/mol, Cl about -349 kJ/mol. In both pairs the period-3 element (S, Cl) is more negative than the period-2 element (O, F).
Chlorine has a strong effective nuclear charge to pull in an electron, plus a spacious 3p shell that avoids the crowding problem faced by fluorine. This combination makes it release the most energy on gaining an electron.
The idea of small-size effects appears in many period-2 anomalies, but the specific 'less negative electron gain enthalpy' point is mainly discussed for O/F versus S/Cl. Ionization enthalpy has its own anomalies (like N being higher than O due to half-filled stability).
Yes. Fluorine is the strongest oxidising agent among halogens. Electron gain enthalpy in the gas phase is only one factor; in solution, F's small size gives a very high hydration energy, so it is still extremely reactive.
Remember one line: Cl > F for electron gain enthalpy magnitude, and S > O too. The tiny 2p shell of the top element repels the extra electron. If an option calls F the 'most negative', it is a trap.
