a non-bridging oxygen

A non-bridging oxygen is a dead end in the glass network. Where a bridging oxygen joins two network cations, a non-bridging oxygen is bonded to only one, leaving its other bond dangling with a negative charge that is soothed by a nearby modifier cation such as sodium or calcium. In a picture, it is a Si-O with a loose end pointing into a pocket that holds an Na+ ion. Every non-bridging oxygen is a broken rung, a place where the network stops rather than continues.

Non-bridging oxygens are made by network-modifiers, one bond-break at a time. Add a unit of Na2O to silica and its extra oxygen slices an Si-O-Si bridge in half, turning one bridging oxygen into two non-bridging ones, with two Na+ ions parked alongside to keep the charge balanced. So the arithmetic is simple: each modifier oxygen creates two non-bridging oxygens. The number of non-bridging oxygens per network cation, often written NBO/T, is the standard yardstick of how depolymerized, how chopped-up, a glass network has become.

This one number quietly controls a whole family of properties, and it is worth being honest about the trade. More non-bridging oxygens mean a less connected network, so the glass has lower viscosity and a lower softening temperature (easy to melt and shape), but also higher thermal expansion (worse thermal-shock resistance) and lower chemical durability, because those weak, charged dead-ends are the first sites water and acids attack. When people say soda-lime glass is cheaper and easier but less durable than fused silica, non-bridging oxygens are the reason in a single phrase.