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Why is glass less dense than a corresponding crystalline material?

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Glass is typically less dense than its corresponding crystalline material because of its random atomic arrangement, which results in a less efficient packing of atoms compared to the highly ordered structure of a crystal.1

In a crystalline solid, atoms are arranged in a regular, repeating array with long-range order, allowing for a dense, compact structure. In contrast, the amorphous structure of glass lacks this long-range periodicity. This random atomic order creates larger interstitial spaces, or holes, between the atoms. These spaces collectively constitute what is known as "free volume," which is the primary reason for the lower density of glass compared to a crystal.21

It is important to note that this density difference is not universal for all glass compositions. While pure silica glass is less dense than its crystalline counterpart (low-cristobalite), the addition of network-modifying ions—such as sodium or calcium—can fill these interstitial spaces, thereby increasing the density of the resulting glass.1