An I.C. may be bonded to a hetero atom by a single or an aromatic bond.
All hydrogens bonded to I.C.s are fragments.
These two kinds of fundamental fragments are the most important members of the non-polar class. A fragment is any atom or group of atoms bounded by Isolat ing Carbon atoms, and all except hydrogen are considered polar. A fragment may have many internal bonds but those connecting it to I.C.s are called 'valence bonds'. Valence bonds are most often single, but can be aromatic, as in the case of the N fragments in pyrimidine shown above. Each hydrogen in methane is a fragment, but the hydrogens in formaldehyde are not because the carbon to which they are bonded is not isolating.