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).