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