Video Transcript
Which of the following elements has
an atomic radius larger than that of aluminum? (A) Chlorine, (B) sodium, (C)
sulfur, or (D) silicon.
This question is asking us to
compare the atomic radii of different elements. The atomic radius of an element is
the distance from its nucleus to its outermost electrons. Scientists can measure the atomic
radius by taking half the distance between the two nuclei of adjacent atoms bonded
together. To answer this question, we need to
take a look at the periodic table, specifically at period three, which contains all
five of the elements mentioned in the question.
In order to answer this question,
we need to know how atomic radius changes across a period. Once we know this trend, we can
find which of the answer choices has the largest atomic radius. Let’s compare aluminum and silicon
to visualize this change. Since we don’t know what size they
are yet, we’ll start by drawing them as the same size. The first difference we can notice
is that when we move to the right in a period, the number of protons increases. Silicon has one more proton than
aluminum. With more protons in the nucleus,
there’s a stronger positive charge there and thus a stronger inward pull on the
electrons. As a result, there’s also a smaller
atomic radius.
So, the trend is clear. If more protons means a smaller
atomic radius, then the atomic radius decreases to the right and increases to the
left. If we wanna find the element with
an atomic radius larger than that of aluminum, we want to find the element to the
left of aluminum on the periodic table. In this case, that’s sodium. Sodium has fewer electrons than
aluminum and therefore a weaker inward pull on its electrons, which results in a
bigger atomic radius. Which of the following elements has
an atomic radius larger than that of aluminum? That’s answer (B) sodium.
