# Order of basicity of halides [duplicate]

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.)

• There are so many competing effects going on here. Also, how these measurements are made. Are they gas phase? In aqueous solution? – Zhe Jul 15 '17 at 1:53

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 $\ce{Cl}$.