next two, Ca and Sr, have close packing at room temperature, but
change to body centering above 600°C; the largest alkaline earth,
Ba, always uses the open bcp structure. The metals in Groups IIIA
and IVA have a more-or-less distorted cp structure because the forces
between atoms are more covalent than the forces in Group IA and
IIA metals. The borderline with nonmetals is not far off.
The first nonmetal in Row 2, boron, is encountered at the top of
Group IIIA. It has a complex three-dimensional covalent cage structure.
The no-man's-land between metals and nonmetals is especially dramatic
in Groups IVA, VA, and VIA, with the lighter elements being nonmetals,
the heavier elements at the bottom of the table metals, and a transition
zone separating them. In this zone, one element often has two different
crystal forms, or allotropes, one metallic and the other
nonmetallic. The nonmetallic allotrope is held together by covalent
bonds with all electrons localized, whereas the metallic form usually
will show the dark color and metallic luster that indicate the presence
of mobile electrons.