The following figure shows
information for five alkaline Earth metals. Which of these elements is expected
to have the largest atomic radius? (A) Beryllium, (B) magnesium, (C)
calcium, (D) strontium, (E) barium.
Atomic radius is a measure of how
large atoms of an element are. There are two important periodic
trends to know for atomic radius. Periodic trends can occur across a
period, up or down a group, or from one corner of the periodic table to the
First, going left to right across a
period tends to cause slight decrease in the atomic radius. This is because as we go left to
right, we increase the number of protons in the nucleus and shielding is
approximately constant. So the effective nuclear charge
increases across a period, pulling the outermost electrons a little closer and
slightly decreasing atomic radius.
More significantly though, as we go
down a group, top to bottom, atomic radius increases significantly. This is because as we go down the
group, each element contains another full shell of electrons. This means that shielding increases
significantly. And it also means that the
outermost electrons are in orbitals further from the nucleus. So going down a group, the elements
at the bottom of the group will have the largest atomic radius.
In the case of these elements,
barium is at the bottom of the group. So the correct answer is (E). Barium is the correct answer.