Well, it's exactly what you think it is. If you think about atom as being a sphere, then our radius begins at the center of our nucleus and ends at the outermost electron shell. So, for this atom, our radius is the black line. So I want to ask the question what happens to our italic radius as we go from left to right across the period on our periodic table? For example, let's take the following period. Let's begin with lithium and go all the way up to four.