Multiple choiceQuestion 1 · Recall
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By the end you can describe the relative reactivity of the halogens as oxidising agents, write equations for their displacement reactions, and explain the trend down Group 17.
Oxidising agent
A substance that oxidises another species by accepting electrons from it. The oxidising agent is itself reduced in the process.
A halogen atom has seven outer electrons, one short of a full shell. The molecule is electron-poor, so it reacts by gaining electrons:
Gaining electrons is reduction. So a halogen acts as an oxidising agent: it takes electrons from something else and oxidises it.
The more readily a halogen accepts electrons, the stronger its oxidising power.
Oxidising power decreases down Group 17:
Fluorine is the strongest oxidising agent of the group; iodine is the weakest.
Here is why. Down the group, each atom is larger and the outer shell is further from the nucleus. There are also more inner shells shielding the outer shell.
Both effects weaken the nucleus's pull on an incoming electron. A fluorine atom grabs an extra electron much more strongly than an iodine atom does, so fluorine oxidises other substances more powerfully.
Picture the nucleus as a magnet and the incoming electron as a paperclip. A small atom puts the paperclip close to the magnet; a large atom makes it land further away, with more shielding in between.
So fluorine holds on to the incoming electron most strongly, and iodine least strongly.
Where the analogy stops: the incoming electron also has to overcome repulsion from the atom's own electrons. That is exactly what the shielding idea describes.