So let's look at the following two atoms. Let's look at lithium and let's look at sodium. Sodium is right below lithium on the same group on the periodic table. So notice that we have two energy levels, one S and two S for the lithium, while the sodium has not two, but three energy levels. One S, two F and three S.
Now, this addition of the three S means that our atom will grow in size, will enlarge. Where this guy, the atomos guy, is the three F. Shell.