Cyclic aromatic compounds having one or more heteroatoms are termed heterocyclic aromatic compounds. Five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole, having nitrogen, oxygen, and sulfur as heteroatoms. In these planar compounds, all atoms are sp2 hybridized, including the heteroatom. As evident from the π bonding structure of pyrrole, one π electron from each carbon and one lone pair of electrons from the heteroatom occupy the unhybridized 2p orbitals. These five orbitals overlap, forming a closed-loop of 6 π electrons, making pyrrole aromatic. In furan and thiophene, the additional lone pairs of electrons occupy an sp2 hybrid orbital and are not part of the π system. Imidazole has two nitrogen atoms. The lone pair on one of the nitrogens is part of the aromatic sextet. The lone pair on the second nitrogen occupies an sp2 hybrid orbital and is not involved in the π system.