Video: Comparing Reactivity Trends of Halogens in Substitution Reactions

In the substitution reaction of methane and a halogen, which of the following halogens is the most reactive? [A] Chlorine [B] Iodine [C] Bromine [D] Fluorine


Video Transcript

In the substitution reaction of methane and a halogen, which of the following halogens is the most reactive? Chlorine, Iodine, Bromine, or Fluorine.

Let’s quickly review what we mean by a substitution reaction. A substitution reaction is one in which one atom or group on a reactant is swapped for something else. So the example given here is the reaction of methane with a halogen. Let’s use methane plus bromine as an example. This reaction is carried out by using UV light and results in the production of methyl bromine and hydrogen bromide.

You can see that what we’ve done here is we’ve substituted one of the hydrogens on the methane with one of the bromine atoms in Br2. And the same sort of process would happen with the other halogens.

What this question is asking us, remember, is which of these halogens is the most reactive. When you’re being asked to compare the reactivity of elements in the same group of the periodic table, we can actually use the periodic table to work out which is the most reactive.

The halogens can be found in group 17 on the right-hand side of the periodic table. When we look at our periodic table, we can see that fluorine is at the top, followed by chlorine, then bromine, and iodine at the bottom. We’ll ignore the elements that come below iodine for this question.

So what happens to the reactivity of these elements as we go up the group? Can you remember? Hopefully, you’ve remembered that the reactivity of an element increases the higher up the group it is. This means that bromine is more reactive than iodine. Chlorine is more reactive than both bromine and iodine. But fluorine is the most reactive of the halogens.

So the correct answer to this question is fluorine. If you’re interested as to why the reactivity increases as we go up the group, have a think about the number of core electrons that each element has. However, we don’t need to know these details at this stage.

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