A
similar borderline is crossed by the elements of Group VA. Having
three unpaired outer electrons, these elements all prefer to make
three bonds to other like atoms. Nitrogen is so small that one atom
can form a triple bond to another atom, thereby making
gas molecules. Phosphorus is too large for triple bonds, so it makes
single bonds to three other P atoms instead, at the corners of a
tetrahedral molecule
(see right). These
tetrahedra are found in the vapor, liquid, and in solid white phosphorus.
Black phosphorus also has each atom linked to three neighbors, but
in sheets like a puckered version of graphite. The black color is
a clue to the presence of mobile electrons within each of the stacked
sheets, and this allotrope is more metallic. Red phosphorus is a
third allotrope of uncertain structure, possibly an amorphous mixture.
Arsenic and antimony also have two allotropes, very unstable yellow
nonmetallic forms, with separate molecules of
and tetrahedra,
and metallic allotropes with stacked sheets like those of black
phosphorus. The color of this metallic form changes from black in
P, to gray in As, to a gunmetal blue sheen in Sb. Bismuth, at the
bottom of the group, has only the sheet structure with a white metallic
luster.