Define here the bond properties related with its charge distribution.
Covalency
Covalency is a measure of the percentage ionic/covalent character 
of a bond. Currently no default value is provided.
Order
Order represents the chemical bond order. Although any positive 
number is allowed, the usual values are 
1.0 (single bond, the 
default), 
2.0 (double bond), 
3.0 (triple bond) and 
1.5 (aromatic or delocalised bond). 
Single, double and triple bonds are rendered in 
Wired mode,
as single, double and triple rasterized lines. In 
Solid mode,
single bonds are represented by a cylinder (the sigma bond), double 
bonds have two additional curved volumes on each side (the pi bond), 
 and triple bonds have two more curved volumes on each side (the
second pi bond). 
Currently, the orientation of the curved volumes in solid secondary 
bonds is arbitrary. Currently, aromatic bonds are handled as normal 
bonds, according to its bond order, so bonds with order equal to 1.5 
are handled as double bonds.