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The order of basicity of halides is $\ce{F- >Cl- > Br- > I-}$

My teacher explained this on the basis that the smaller is the ion, the greater is the charge/size ratio, hence it more unstable and more likely to gain a proton.

However if this is the case, why is the magnitude of electron gain enthalpy in the order $\ce{Cl- > F- > Br- > I-}$ and not $\ce{F- < Cl- < Br- < I-}$ since we are calling $\ce{I-}$ the most stable and so on?

(The reaction is going on in an aqueous solution.)

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    $\begingroup$ There are so many competing effects going on here. Also, how these measurements are made. Are they gas phase? In aqueous solution? $\endgroup$
    – Zhe
    Commented Jul 15, 2017 at 1:53

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The electron gain enthalpy you've mentioned is measured in the gas phase, not in aqueous solution like the order of basicity.

When you go down on a group, two important properties are increased: nuclear charge and atomic radius. However, the increase in atomic radius is much more pronounced than of the nuclear charge. Considering this, as you go down on the halogens group the electrons added feel less attracted to the nucleus and, therefore, the electron gain enthalpy become less negative.

The order is only apparently changed for $\ce{Cl-}$ and $\ce{F-}$ due to fluorine small size. The additional electron would be added to a "small" $\ce{2p}$ orbital in fluorine (and there would be more electron-electron repulsion) while in the chlorine atom it would be added to a "big" $\ce{3p}$ orbital that can better accommodate the additional charge, so the enthalpy is less negative for the electron gain in $\ce{F}$ then for $\ce{Cl}$.

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  • $\begingroup$ So is it possible to measure electron gain enthalpy in aqueous solutions, and what is (are) the factors that change the order? $\endgroup$
    – User
    Commented Jul 15, 2017 at 3:05
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    $\begingroup$ @InternetGuy That's fine, but in solution, entropic effects would be very much relevant. Solvation is the big factor that makes this a complicated problem. $\endgroup$
    – Zhe
    Commented Jul 15, 2017 at 3:09
  • $\begingroup$ Maybe there is a way of measuring it in aqueous solution that I'm not aware. But I can guarantee it won't be as easy as in the gas phase. In solution there are other interactions that would affect the enthalpy measured, not to mention the difficulty to produce elementar X atoms in aqueous solution. Just the water-halogen interaction in aqueous solution would make your measurement much harder. $\endgroup$
    – Raul Luciano
    Commented Jul 15, 2017 at 3:13
  • $\begingroup$ So basically there is no exact theoretical reason for the observed order of basicity? $\endgroup$
    – User
    Commented Jul 15, 2017 at 8:10
  • $\begingroup$ @User , Ofcourse there is. In an aqueous solution, although halide ions are formed , There is no actual gain or loss in electron. Therefore HF dissociating into H+ and I- cannot be considered that Iodine was physically forced to accept one electron in its orbitals. $\endgroup$
    – Jdeep
    Commented Aug 29, 2020 at 4:07

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