Considering the double-bond carbons separately, look at the two atoms directly attached to each and rank them according to atomic number. An atom with higher atomic number receives higher priority than an atom with lower atomic number. Thus, the atoms commonly found attached to a double bond are assigned the following order. Note that when different isotopes of the same element are compared, such as deuterium (2H) and protium (2H), the heavier isotope receives priority over the lighter isotpe.