In this reaction, the oxidation state of Li changes from 0 to -1. In its elemental form, lithium has a neutral charge, meaning its oxidation state is 0. When it reacts with hydrogen to form lithium hydride (LiH), each lithium atom gains one electron from hydrogen, resulting in an oxidation state of -1. This is because hydrogen has an oxidation state of +1 in most compounds, and in order to balance the charges, lithium must take on a negative oxidation state.