Now what happens as we move, for example, from lithium to fluorine? We already said that our effective nuclear charge tends to increase as we go from left to right. So fluorine has the highest force. In other words, the protons found in the nucleus. Pull those electrons on the outermost electron shell with a lot of force, much more force than lithium or beryllium or boron or carbon. And that means it's going to require much more energy to pull those outermost electrons off.