hydrogen and potassium ((stage 1)). In situ adsorption on free-standing graphene and intercalation in bilayer graphene of the alkali metals K, Cs, and Li was observed by means of low-energy electron microscopy. Different from other alkali metals, the amount of Na intercalation is very small. Quantum-mechanical calculations show that this originates from a quite general phenomenon: among the alkali and alkaline earth metals, Na and Mg generally have the weakest chemical binding to a given substrate, compared with the other elements in the same group of the periodic table. The phenomenon arises from the competition between trends in the ionization energy and the ion–substrate coupling, down the columns of the periodic table. However, considerable Na intercalation into graphite can occur in cases when the ion is wrapped in a solvent shell through the process of co-intercalation. A complex magnesium(I) species has also been intercalated into graphite. Graphite bisulfate, perchlorate, hexafluoroarsenate: oxidized carbons The intercalation compounds graphite bisulfate and graphite perchlorate can be prepared by treating graphite with strong oxidizing agents in the presence of strong acids. In contrast to the potassium and calcium graphites, the carbon layers are oxidized in this process: 48 C + 0.25 O2 + 3 H2SO4 → [C24]+[HSO4]−·2H2SO4 + 0.5 H2O In graphite perchlorate, planar layers of carbon atoms are 794 picometers apart, separated by ions. Cathodic reduction of graphite perchlorate is analogous to heating , which leads to a sequential elimination of . Both graphite bisulfate and graphite perchlorate are better conductors as compared to graphite, as predicted by using a positive-hole mechanism. Reaction of graphite with affords the salt . Metal halide derivatives A number of metal halides intercalate into graphite. The chloride derivatives have been most extensively studied. Examples include (M = Zn, Ni, Cu, Mn), (M = Al, Fe, Ga), (M = Zr, Pt), etc. The materials consists of layers of close-packed metal halide layers between sheets of carbon. The derivative exhibits spin glass behavior. It proved to be a particularly fertile system on which to study phase transitions. A stage n magnetic graphite intercalation compounds has n graphite layers separating successive magnetic layers. As the stage number increases the interaction between spins in successive magnetic layers becomes weaker and 2D magnetic behaviour may arise. Halogen- and oxide-graphite compounds Chlorine and bromine reversibly intercalate into graphite. Iodine does not. Fluorine reacts irreversibly. In the case of bromine, the following stoichiometries are known: