The equals sign =, is a common convention for
representing a double bond connection between the adjacent carbon atoms in the
formula. Similarly, the dash -, is used to represent a single bond
connection. In most formulas, the dash is usually omitted as the single
bond connectivity between carbon atoms is understood. The dashes are used
in the formula for Urea, for example to show the connectivity between the 2
amino (NH2) groups and the central carbon atom. As a rule,
Carbon has 4 valence electrons available for binding and each carbon atom will
have 4 bonds in a compound.
In Urea, the central carbon atom has a single bond to each nitrogen, and a
double bond to the oxygen atom. Here again, the equals sign representing
the double bond is omitted as oxygen routinely forms one double bond or two
single bonds to adjacent atoms.