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Question Video: Identifying the Element with the Largest Atomic Radius Chemistry

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.

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Video Transcript

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 next.

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.

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