What are the patterns/trends that you see in the atomic radii?

Looking at the table above notice from lithium to cesium, going down the group, the atomic radius increases. Also notice that going across a period the atomic radius decreases. How do we understand these two trends?

We might argue that going from hydrogen through the elements that the number of electrons increase. Since electrons repulse each other due to the like charges, that the size of the atom will increase. The more electrons the larger the atom. We might be able to use that arguement to explain the trend within a group, but when we look at the atomic radius trend going across a period the radius decreases.

So we could argue that going from hydrogen thtough the elements the number of protons increases. Since electrons are attracted to protons we might expect that the more protons the smaller the radius. That might explain the trend in atomic radii going across a period but it does not explain the trend going down a group. Cesium has a large number of protons, yet it is one of the largest atoms, not the smallest.

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