In this lecture, we're going to talk about four important periodic trends atomic radius, ionization energy, electronegativity, and electron affinity of atoms. Now, let's begin with atomic radius. So what is an atomic radius? 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.