Furan is a heterocyclic organic compound.
Furan is simply a five-membered with the molecular formula ##C_4H_4O##. More specifically, furan is a heterocycli compound because its ring contains two types of atoms – carbon and oxygen.
Furan’s five-membered ring is actually comprised of four carbon atoms and one oxygen atom. The oxygen is bonded to the ring via two single bonds. Two of the four ring carbons are bonded via double bonds.
In addition to this, each carbon atom has one hydrogen atom attached. Furan’s looks like this
Because the lone pairs of electrons are allylic, i.e. they are located next to the double bonds, furan can exhibit
As you an see, one of oxygen’s lone pairs of electrons is actually delocalized in the ring, which is why furan is considered an aromatic compound (I won’t go into too much detail about why that is). This implies that the actual structure of furan can best be described using a hybrid model that looks like this