Reactivity in periodic table

[Anithadhruvbud]

Amoung three elements F,CL,Br which is more reactive? I guessed it to be bromine because it has three shells and hence when we go down the group on the basis of their shells bromine will easily lose electrons.
But the book says it is fluorine since it can easily gain electrons.And how can that be? Shouldn't we classify something as reactive on the basis of electropositivity?
Looks odd because another answer to a question similar to this answered on the basis of electronegativity.
What exactly is the criteria I should use here?

 

[mfb]

All three elements "prefer" to accept an additional electron, instead of donating electrons: they just need one more for a noble gas configuration. And fluorine does so much better because it just has two shells.

 

[Anithadhruvbud]

All three elements "prefer" to accept an additional electron, instead of donating electrons: they just need one more for a noble gas configuration. And fluorine does so much better because it just has two shells.

Got it! Thank you! But what if there was a mixture of elements which preferred to gain and lose electrons? In that case,which is more reactive?
It is justified if all three preferred to lose electrons because the one which has more shells will be more reactive,just opposite to the case of non metallic nature,isn't it?

 

[mfb]

It doesn't make much sense to compare reactivity of materials on the opposite side of the spectrum, e. g. caesium with fluorine. In general they will react with different things.