What is the correct answer to the electron gain enthalpy NEET 2026 question?
Option C is correct: the supplied official solution ranks electron gain enthalpy magnitudes, not signed values. The electron gain enthalpy NEET 2026 question belongs to Classification of Elements and Periodicity in Properties.
“Which sequence correctly ranks the electron gain enthalpy of these four halogens?”
This question bank rates the MCQ medium and sets an expected solving time of 45 seconds. That is the bank’s target, not a measured student average. The deciding comparison is chlorine versus fluorine: chlorine releases more energy when it accepts an electron.
What does electron gain enthalpy measure, and what does its sign mean?
Electron gain enthalpy is the enthalpy change when an isolated gaseous atom accepts an electron. For all four halogens in this question, first electron gain is exothermic: energy is released, so the enthalpy change is negative.
The first-electron-gain process is:
A more negative enthalpy means more energy is released. Its magnitude is larger, but its signed numerical value is smaller. Confusing these statements reverses the inequality signs even when the element sequence is correct.
The supplied official solution interprets the choices as magnitude orders. Read “comes before” as “releases more energy on accepting the first electron,” not “has a numerically greater enthalpy.”
- Decreasing magnitude: put the strongest energy release first.
- Increasing signed value: put the most negative enthalpy first.
These instructions give the same element sequence here, but require opposite inequality signs.
Why does chlorine come before fluorine in the worked solution?
Chlorine releases more energy because its incoming electron enters a larger 3p subshell, with less crowding than fluorine’s compact 2p subshell. Chlorine therefore comes before fluorine in magnitude, despite fluorine being smaller.
Step 2: identify where the electron goes. Halogens accept an electron into their outer p subshell, completing it. A p subshell contains three orbitals, each holding at most two electrons:
Step 3: compare crowding. In fluorine, the incoming electron experiences strong electron–electron repulsion in the compact 2p subshell. Chlorine’s larger 3p subshell reduces this crowding, making electron addition more favourable.
Therefore, chlorine has a more negative first electron gain enthalpy than fluorine. Smaller atomic size alone does not decide the comparison.
Step 4: complete the order. Down the heavier halogens, increasing size and shielding reduce attraction for the incoming electron, generally reducing electron gain enthalpy magnitude. Retain the fluorine–chlorine exception rather than forcing a smooth group trend.
The official magnitude order selects C:
The corresponding signed-value order is:
For the bank’s 45-second target: identify the magnitude convention, apply the chlorine–fluorine exception, complete the order, then match C.
Why is option A wrong even though fluorine is the smallest halogen?
Option A fails because it treats atomic size as the only deciding factor. The faulty method places the smallest halogen first, then assumes electron gain enthalpy magnitude decreases smoothly down the group:
That method accounts for attraction to the incoming electron but misses repulsion from electrons already present. Fluorine’s compact 2p subshell creates enough crowding to change its comparison with chlorine.
Before applying a smooth group trend, compare the receiving subshells of the first two elements. Here, that means compact 2p versus larger 3p.
Electronegativity is not a shortcut. Fluorine being the most electronegative element describes its attraction for shared electrons in a bond; it does not establish the enthalpy change for electron addition to an isolated gaseous atom.
How can I apply this reasoning to three related practice questions?
Use two checks: identify whether the question asks for magnitude or signed value, then compare the receiving subshells. These are newly written related practice questions, not verified past-paper questions.
How do I arrange the halogens in increasing numerical electron gain enthalpy?
Chlorine comes first, followed by fluorine, bromine and iodine. Increasing numerical value puts the most negative enthalpy first.
Related practice question 1: “Arrange F, Cl, Br and I in increasing numerical value of their first electron gain enthalpies.”
All four values are negative, and chlorine’s is the most negative. Do not copy the greater-than symbols from the original options: those represent decreasing magnitude, while this question asks for increasing signed value.
Between oxygen and sulfur, which has the more negative first electron gain enthalpy?
Sulfur has the more negative value. Its larger 3p receiving subshell has less crowding than oxygen’s compact 2p subshell.
Related practice question 2: “Between oxygen and sulfur, which has the more negative first electron gain enthalpy, and why?”
Oxygen’s incoming electron experiences greater electron–electron repulsion in the compact 2p subshell. Reduced crowding in sulfur’s 3p subshell makes electron addition more favourable. Apply the fluorine–chlorine reasoning: compare the receiving subshells rather than selecting the smaller atom automatically.
Does fluorine’s higher electronegativity imply a more negative electron gain enthalpy than chlorine’s?
No. Electronegativity concerns attraction for shared electrons in a bond; electron gain enthalpy concerns adding an electron to an isolated gaseous atom.
Related practice question 3: “Does fluorine’s higher electronegativity imply a more negative electron gain enthalpy than chlorine’s?”
For electron addition, repulsion in fluorine’s compact 2p subshell matters. Chlorine’s larger 3p subshell makes addition more favourable, giving chlorine the more negative value. Before choosing an order, underline “magnitude” or “numerical value”, then check the receiving subshell.
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Frequently asked questions
What is the correct answer to the electron gain enthalpy NEET 2026 question?
Option C is correct under the supplied official solution’s magnitude convention: Cl > F > Br > I. This ranks the energy released on first electron gain, not the signed numerical enthalpy values.
Why does chlorine have a more negative electron gain enthalpy than fluorine?
The incoming electron enters chlorine’s larger 3p subshell, where it experiences less electron–electron repulsion than in fluorine’s compact 2p subshell. Electron addition therefore releases more energy in chlorine, making its first electron gain enthalpy more negative.
What is the increasing numerical order of electron gain enthalpy for halogens?
The increasing signed-value order of first electron gain enthalpy is Cl < F < Br < I. All four values are negative, so chlorine’s most negative value comes first. The decreasing magnitude order uses the same element sequence but opposite inequality signs.
Does higher electronegativity mean more negative electron gain enthalpy?
No: electronegativity describes attraction for shared electrons in a bond, while electron gain enthalpy describes electron addition to an isolated gaseous atom. Fluorine is more electronegative than chlorine, but crowding in fluorine’s compact 2p subshell makes its first electron gain enthalpy less negative than chlorine’s.