Silica glasses are composed of multi-oxides, apart from the major component silica. Though it is a general practice in the industries to prepare glasses at specific oxide ratios, the composition rule is largely missing, complicated by the implication of multi-oxides. Necessarily, their interpretation is rooted in chemical units, on which the specific compositions depend. However, in silica glasses the inter-atomic bonding network is continuous and there is no weak bonds, like the inter-molecular ones in molecular compounds, to define molecular entities that carry the chemical information of the materials. As the first stage towards understanding the composition rule, the present paper introduces a new method, so-called the cluster-plus-glue-atom model, to unveil the molecule-like structural units of the glass-relevant oxides. It is pointed out that their respective contributions to the construction of glass networks originate from their characteristic cluster structures, and from which molecule-like structural units are proposed that represent the smallest structural units of these oxides. Oxides participating in the glass network formation mainly present triangular or tetrahedral clusters which are required for a three-dimensional glassy network. For example, the basic network former SiO₂ is formulated as [Si-O₄]Si and contains 32 valence electrons. The intermediate oxides are characterized by the simultaneous formation of both octahedra and tetrahedra. The network modifiers present mainly cubes and octahedra. It is confirmed that the molecule-like structural units of the glass-formation oxides all meet octet rule (that is, the total number of valence electrons contained in each structural unit is an integer multiple of 8), just like common molecules. The proposed concept of molecular structural units sheds a new light on understanding the composition rule of silicate glasses and can eventually solve the long-standing problem of composition design of silica glasses.