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How do network-modifying ions change the atomic structure of silicate glass?

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In silicate glass, the atomic structure is defined by a three-dimensional network of SiO4 tetrahedra connected at their corners by bridging oxygen atoms. Network-modifying (NWM) ions, such as alkali or alkaline-earth metal ions, alter this structure by breaking the continuity of the network.123

When NWM ions are introduced, they react with bridging oxygens to create nonbridging oxygens—oxygen atoms bonded to only one silicon atom. The remaining negative charge on these nonbridging oxygens is satisfied by the NWM cation, which occupies an interstitial space adjacent to the tetrahedron.1

This process has several effects on the glass structure and properties:
* Connectivity: The formation of nonbridging oxygens lowers the overall connectivity of the silicate network.2
* Viscosity: Reduced network connectivity leads to a decrease in the viscosity of the glass melt.2
* Openness and Density: The presence of NWM ions influences the openness of the structure; networks with fewer NWM ions are less dense and more permeable to gases.2
* Durability: Higher concentrations of mobile NWM ions, such as alkali ions, generally decrease the material's chemical durability and electrical resistivity.2