Fluorine has nine protons in its nucleus. While two electrons are found in the inner shell, seven electrons are found on the outer shell. And that means because we have a higher nuclear or effective nuclear charge, we have a higher force. In other words, because we have nine protons in our nucleus and seven electrons on our outer shell, our force with which our nucleus pulls those electrons is much greater than the force with which these three protons are pulling a single electron. And so that means if we place some arbitrary electron equidistant between these two atoms what? We see that this fluorine will pull this electron with much more force than this guy.