10. Playing with a Full Deck:
       The Periodic Table
  Previous PageNext Page
       The Structures of the Elements

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.

Melting points provide information about the forces between atoms in a solid. Melting points of metals increase with the number of bonding electrons per atom (Li to Be; K to Ca), and decrease with atomic size for atoms with the same number of bonding electrons (Li to Cs). Hardness and melting point increase with greater covalent bonding between atoms (Li to C; Pb to C). The change from infinite three-dimensional frameworks of covalent bonds to indisvidual, covalently bonded molecules causes a drastic drop in melting points (C to N; Si to P).
  Page 15 of 63 HomeGlossary