Graphite is an established solid lubricant and behaves as a semiconductor in a direction perpendicular to its planes. Most of its chemistry is nonmetallic; it has a relatively high ionization energy and, compared to most metals, a relatively high electronegativity. Carbon can form anions such as C (methanide), C 2 (acetylide), and C 4 (sesquicarbide or allylenide), in compounds with metals of main groups 1–3, and with the lanthanides and actinides. Its oxide CO 2 forms carbonic acid H 2 CO 3.