a network former
Think of building a big, springy jungle gym. The essential pieces are the strong corner joints that actually hold the whole frame together — without them there is no structure, just loose bars. In an oxide glass, a network former is exactly that: the atom whose strong, directional bonds knit the continuous random network into a self-supporting three-dimensional framework. Silicon in silica glass is the classic example; boron, phosphorus, and germanium play the same role in their glasses. Take the formers away and you have no glass network at all.
What makes an oxide a network former is chemistry with a geometric consequence. A former is a small cation with a high charge-to-size ratio that bonds to oxygen strongly and directionally, taking a low coordination number — usually three or four oxygens arranged as a triangle or a tetrahedron. These small units link only at their corners, each shared oxygen bridging two formers, and that corner-sharing is what lets the network flex to random angles while staying fully connected. Silicon dioxide (SiO2), boron trioxide (B2O3), phosphorus pentoxide (P2O5), and germanium dioxide (GeO2) all satisfy this and readily form glasses on their own; Zachariasen's rules were essentially a checklist for which oxides make good formers. Because the former's bonds are strong, a pure former glass like silica has a very high softening temperature — one reason fused silica is so hard to work.
The concept matters because almost every practical glass is designed by mixing formers with other oxides. The former supplies the backbone; other ingredients tune it. This immediately sets up the key partnership of glass chemistry: network formers build the network, while network modifiers break it, and control of that balance is how glassmakers set melting point, viscosity, and durability. An honest nuance: 'former' versus 'modifier' is not an absolute label but depends on the glass. Aluminium and some others are intermediates — they can act as formers in one composition and modifiers in another — so the classification describes a role in a particular glass, not a fixed property of the element.
Pure fused silica is made of nothing but the network former SiO2: every silicon corner-shares four oxygens, every oxygen bridges two silicons, and the whole solid is one giant connected network. That is why it is extraordinarily strong and heat-resistant — and why it must be melted above 1700 degrees Celsius to work, until modifiers are added to tame it.
A network former's strong, low-coordination, corner-sharing bonds build the glass backbone.
Former versus modifier is a role within a given glass, not a fixed elemental property. Intermediates such as aluminium can form the network in one composition and modify it in another, so always ask 'former in which glass?' rather than treating the label as absolute.