And we see that as we go from left to right, across the period, from Lithium to Fluorine, our Electronegativity increases. So let's examine why. Let's look at atomic structure and the electron configuration of lithium and compare to that of fluorine. Now, lithium has three electrons and three protons. So its nucleus is composed of only three protons, while its inner shell is composed of two electrons, and its outer shell is composed only of a single electron. Now let's look at the fluorine.