How has iodine gained electrons?

$$\ce{Cr2O7^2-(aq) + 14H+(aq) + 6I-(aq) -> 2Cr^3+(aq) + 3I2(aq) + 7H2O(l)}$$


Two chromium atoms gain 3 electrons each. They go from $0$ to $3+$


Similarily, 6 iodine atoms go from $-1$ to $0$, thus gaining one electron. However, my textbook says that iodine has lost electrons. How?


It went from negative to positive, i.e. from $-1$ to $0$, just like chromium from $0$ to $+3$.




The mistake here is understanding what happens to the dichromate ion. Chromium has gone from the +6 to the +3 oxidation state. Its oxidation state has decreased by three therefore it has *gained* three electrons in the process: $$\ce{Cr2O7^{2-} + 14H+ + 6e- -> 2Cr^{3+} + 7H2O}$$


Iodine has gone from the -1 to the 0 oxidation state. Its oxidation state has increased by one, *losing* one electron in the process. $$\ce{2I- -> I2 + 2e-}$$


Putting these two half equations together gives you the overall equation which you started with. $$\ce{Cr2O7^{2-} + 14H+ + 6I- -> 2Cr^{3+} + 7H2O + 3I2}$$


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Two points before we begin:


1. `+` charge means deficiency in electrons.
2. `-` charge means excess of electrons.


Now lets come to the reaction you are talking about.



> 
> Two chromium atoms gain 3 electrons each. They go from 0 to 3+
> 
> 
> 


Moving from neutral to +3 charge means it just lost 3 electrons. (*Electrons have negative charge.*)



> 
> Similarily, 6 iodine atoms go from -1 to 0, thus gaining one electron. However my textbook says that Iodine has lost electrons. How?
> 
> 
> 


Your textbook is right and you're wrong here. Moving from -1 charge to 0 means that an atom just lost an electron (*in other words, it lost the excess negative charge it had and now it is neutral*).


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