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. And that's exactly why as we go across the period from lithium to fluorine, our ionization energy tends to increase. Because as we go this way, we have a higher effective nuclear charge, which means we have a greater force. And also take this into account.