Q.Among the halogens, chlorine has a more negative electron gain enthalpy ($-349\ kJ\,mol^{-1}$) than fluorine ($-328\ kJ\,mol^{-1}$), even though fluorine is the smaller and more electronegative atom. Explain why.

Among F, Cl, Br and I, chlorine has the largest negative electron gain enthalpy, not fluorine.

Electron gain enthalpy is the energy released when an isolated gaseous atom gains an electron. Intuitively, since fluorine is the smallest halogen with the highest effective nuclear charge felt at the surface, one might expect it to have the most negative electron gain enthalpy. However, fluorine's atom is exceptionally small, so its 2p subshell is already compact and crowded with electrons; adding one more electron causes significant inter-electronic repulsion, which reduces (makes less negative) the energy released.

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The electron affinity order is S > Se > O, i.e. O < Se < S.

Within Group 16, electron affinity is expected to decrease down the group as atomic size increases (electron added farther from the nucleus). However, oxygen is an exception: its very small atomic size means the added electron experiences strong repulsion from the already-compact 2p electrons, lowering its electron affinity below that …

Argon has a positive (unfavourable) electron gain enthalpy because its electron shell is already completely filled and stable, so an incoming electron must be forced into a new, higher-energy shell.

Electron gain enthalpy (ΔegH) is the energy change when a neutral gaseous atom gains an electron to form a gaseous anion. Most atoms release energy on gaining an electron (negative ΔegH) because the extra electron is attracted by the effective nuclear charge and fits into an already partly-vacant valence shell.

Check each option:

• Fluorine: has 7 valence electrons (2s2 2p5); gaining one more electron completes a stable octet and is strongly favourable (large negative/exothermic electron gain enthalpy, one of the most negative among elements after chlorine).
• Hydrogen: gaining an electron to form H- is mildly exothermic (small negative electron gain enthalpy), completing the 1s2 configuration.
• Sodium: gaining an electron is exothermic (though weaker than halogens), as it partially fills the 3s subshell. …

Fluorine's very small atomic size causes unusually strong electron-electron repulsion in its compact 2p sub-shell, so chlorine, not fluorine, has the most negative electron gain enthalpy.

Electron gain enthalpy generally becomes more negative across a period and less negative down a group, which would suggest F (top of Group 17) should have the highest negative value. However, fluorine is an exception: its atom is very small, so the incoming electron experiences strong repulsion from the already tightly packed electrons in the compact 2p sub-shell, making the electron gain enthalpy less favourable than expected.

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Among halogens, chlorine (not fluorine) has the highest magnitude of electron gain enthalpy, because fluorine's very small atomic size causes unusually strong electron-electron repulsion in its compact 2p subshell.

Electron gain enthalpy (in kJ/mol, more negative = more energy released = higher magnitude): F = -328, Cl = -349, Br = -325, I = -295.

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Fluorine's electron affinity is actually less than chlorine's, an exception to the usual group trend, because fluorine's very small atomic size causes strong inter-electronic repulsion in its compact 2p subshell.

Going down Group 17 one might expect electron affinity to decrease steadily (as it does from Cl to Br to I), but F is anomalous: its atomic radius is unusually small, so the electron density in the 2p subshell is very high. Adding one more electron to this already-crowded, small shell causes significant electron-electron repulsion, which reduces the ene …

Argon, a noble gas with a stable filled octet, has a positive (unfavourable) electron gain enthalpy.

Electron gain enthalpy is the energy change when an atom in the gas phase gains an electron to form a negative ion. Most atoms release energy on gaining an electron (negative electron gain enthalpy), because the added electron is attracted by the nucleus. However, atoms that already have a particularly stable electron configuration resist accepting an extra electron, since the new electron would have to occupy a higher, less stable orbital/shell — for these atoms, energy must be supplied, giving a positive electron gain enthalpy.

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Chlorine has the highest magnitude of electron gain enthalpy among common elements because its atom is large enough to accommodate an extra electron without severe repulsion, yet has a high effective nuclear charge.

Step 1: Electron gain enthalpy becomes more negative left-to-right across a period and generally less negative down a group -- but fluorine is an exception.

Step 2: Fluorine's very small atomic size means the incoming electron enters a compact 2p subshell already crowded with electrons, causing significant inter-electronic repulsion, which reduces (makes less negative) the energy released. Chlorine's larger 3p subshell has less repulsion, so it releases more energy: EGE(Cl) ~ -349 kJ/mol vs EGE(F) ~ -328 kJ/mol. …

Electron gain enthalpy = the energy released (usually) or absorbed when a gaseous atom gains an electron.

Step 1 -- definition: Electron gain enthalpy (delta-egH) is defined as the enthalpy change that occurs when one mole of isolated gaseous atoms (in their ground electronic state) each accept one electron to form one mole of gaseous anions.

Step 2 -- equation: X(g) + e- -> X-(g), delta-egH. …

Because noble gases have a stable, completely-filled valence shell, forcing an extra electron in requires a lot of energy, giving them the most positive (very high) electron gain enthalpy of any group.

Electron gain enthalpy (ΔegH) is the energy change when an electron is added to a neutral gaseous atom. Across a period, ΔegH generally becomes more negative (more exothermic, more favourable) as effective nuclear charge increases — except for the noble gases at the end of each period.

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