As you look over the representative-element structure table (page 6) again, you can see the gradual diagonal change in properties from lower left to upper right: bcp and then cp metals, an intermediate zone of metallic and nonmetallic allotropes, and finally the covalently bonded discrete molecules of nonmetals. As the number of bonds formed between like atoms in Groups IVA, VA, and VIA changes from four, to three, to two, the metallic allotrope changes from packed atoms, to stacked layers or sheets, to helical chains; and the nonmetallic allotrope changes from a three-dimensional diamond lattice, to tetrahedra, to rings of eight atoms. Only the smallest atoms can form diatomic gases having multiple bonds, but singly bonded diatomic molecules are the rule for the halogens, in Group VIIA.

As we have seen before, the melting point, or the temperature needed to break a solid apart into fluid atoms or molecules, is a convenient measure of the forces between these atoms or molecules. Melting points for the representative elements are plotted on the three-dimensional graph, with the periodic table as the base and temperature along the vertical axis. Along the left edge, the alkali metals in Group IA, having one electron per atom and an open bcp structure, are soft and low-melting. The hardness of the metal and its melting point increase in Group IIA, in which twice as many electrons are available. Al, Ga, and In are lower melting because of the imperfections and openness of their metal structures. Within one group, the heavier metals are softer and lower melting because their atoms are farther apart.