  Topological indices are a class of numerical invariants that predict certain
physical and chemical properties of molecules. Recently, a novel topological
index, named as Sombor index, was introduced by Gutman, defined as
$SO(G)=\sum\limits_{uv\in E(G)}\sqrt{d_{u}^{2}+d_{v}^{2}}$, where $d_{u}$
denotes the degree of vertex $u$.
  In this paper, our aim is to order the chemical trees, chemical unicyclic
graphs, chemical bicyclic graphs and chemical tricyclic graphs with respect to
their Sombor index. We find the first fourteen minimum chemical trees, the
first four minimum chemical unicyclic graphs, the first three minimum chemical
bicyclic graphs, the first seven minimum chemical tricyclic graphs.
