Melting points increase steeply from Li to Be to B to C because of the increasingly covalent character of the forces between atoms. Diamond is the hardest and highest-melting element known. Diagonally across the center of the table, from B to Si to As to Te, the borderline elements with three-, two-, and one-dimensional covalently bonded structures (frameworks, sheets, chains) are harder and have higher melting points because covalent bonds must be broken to disrupt the solid. At the far right of the table, the molecular compounds , , , , and the halogens have lower melting points because, although the molecules are covalently bonded internally, they have only weak van der Waals forces holding them together in the solid.