covalency
Specifies the degree of covalent character of a bond. 
A fully ionic bond might have covalency equal to 
0.0,
while a fully covalent bond might have covalency equal to 
1.0.
Example: <bond ... covalency="0.5"/> (no default)
Allowed values: positive real (optional)
order
Specifies the bond order, 
1.0 for a single bond, 
2.0 
for a double bond, 
3.0 for a triple bond and 
1.5 for 
a benzene aromatic bond.
In 
wired mode, single, double and triple bonds are
described by one, two and three paralel lines, respectively.
In 
solid mode, a single bond is described by a cylinder,
a double bond has two additional cylinders coming slightly
outside of the central cylinder, opposite to each other, 
and a triple bond has two more additional cylinders, all 
four making angles of 90 degrees between them.
Example: <bond ... order="1.0"/> (default)
Allowed values: positive real (optional)