A <diazine> is an aromatic six-membered <heterocyclic compound> whose parent ring contains four <carbon> atoms and two <nitrogen> atoms. The three unsubstituted parent arrangements share formula $\mathrm{C_4H_4N_2}$ but differ in connectivity: <pyridazine> is the $1,2$ arrangement, <pyrimidine> the $1,3$ arrangement, and <pyrazine> the $1,4$ arrangement.<PubChem Compound database by National Center for Biotechnology Information>[Pyridazine CID9259, pyrimidine CID9260 and pyrazine CID9261; ring structures and formulae]{ref}

Why only three? Fix one <nitrogen> at position $1$ on a six-membered ring. The other can be adjacent, separated by one intervening ring atom, or opposite. Reflection identifies the remaining apparent alternatives: positions $6$ and $2$ describe the same adjacent arrangement, while $5$ and $3$ describe the same next-nearest arrangement. Thus the distinct shortest ring separations are one, two and three bonds.

The three <diazines> are <constitutional isomers>, not different orientations of one <molecule>. Rotating a drawing changes its appearance without changing its atom-to-atom connections. Moving a ring <nitrogen> to another connectivity position instead changes which <chemical compound> the drawing represents. This makes the <diazine> family a compact example of why a molecular formula alone does not specify chemical identity.


 Diazine (source code)