So that means when we move one down to potassium, potassium will have a four S. So potassium will be even larger than sodium and definitely larger than lithium. And that's exactly what we see. In other words, as we go down a group, our atomic radius tends to grow in size, while as we go across the period, our atomic radius tends to decrease because our effective nuclear charge of our atom tends to increase. So that's atomic radius. Now let's look at the ionization energy of our atoms. So what is ionization energy?