Molecular Structure of Sodium Metasilicate Anhydrous

The basic formula for sodium metasilicate-water glass-includes two sodium atoms, one silicon atom, and three oxygen atoms. Sodium metasilicate that is produced without any trace of water is "anhydrous." The formula for sodium metasilicate should then be Na₂SiO₃, right? Yes, that's right-however, to dismiss things at that level would be less than inspirational, less than truthful.

Consideration as Subunits

Sodium metasilicate can be considered broken down into two parts. One part is sodium, which is in the form of sodium ions, two of them, each possessing a plus one charge, or Na⁺. The remaining portion of sodium metasilicate, in order to possess charge neutrality, would have to be the metasilicate portion with a minus two charge, or... (SiO3)^-2

Now some atoms tend to form chains, including carbon, boron, and in this instance-silicon. In fact, the silicon and oxygen atoms of neighboring molecules form corner-bonded (sharing of corner oxygen atoms) tetrahedra-a polymeric chain-with apparent formulation (SiO₄), with a silicon atom central in each tetrahedron. Notice that the formulation is only apparent, not real, as two of the four oxygen atoms are shared with adjacent silicon atoms on either side, making it 1/2 + 1 + 1 + 1/2 = 3 oxygen atoms per silicon, even though it would seem to be 4. See the image associated with this article for purposes of visualization.

Structure Upon Dissolution

What is the structure of sodium metasilicate anhydrous upon dissolving in water? Sodium silicate is quite water soluble as well as ionic. Hence the polymeric structure must break down, and the ions are hydrated. The coordination number for sodium has been listed as between 4 and 6. It appears that size of the ion is a determining factor. Since negative silicate ions are far larger, they would logically be surrounded by a far greater number of waters of coordination. Such associated water molecules surrounding an ion would enable the charge to be dispersed across the entire species, thus lessening energy and stabilizing the system.

Other Forms of Sodium Metasilicate

Other forms of sodium metasilicate, such as sodium metasilicate pentahydrate, form stable substructures that are nearly tetrahedral as well. Thus sodium pentahydrate, Na₂SiO₃∙5H₂O, apparently forms a polymeric structure of the form, SiO₂(OH)₂^-2 with water of hydration.¹ Sodium metasilicate nonahydrate responds in similar fashion.

¹ "Structural Inorganic Chemistry," 5th edition, by AF Wells Oxford Science Publications, 1984.

References and Resources:

Brooklyn College - The City University of New York: The Silicon Tetrahedron