Which of the following is NOT a giant covalent substance?

Giant covalent substances, also known as covalent networks, are characterized by their vast, interconnected lattice structures in which atoms are bonded together by covalent bonds in a continuous network. This results in high melting and boiling points, as well as great hardness and other specific physical properties.

Diamond, graphite, and silicon dioxide are all examples of giant covalent substances. Diamond consists of carbon atoms arranged in a highly structured, three-dimensional lattice, giving it extreme hardness. Graphite, another form of carbon, has a layered structure where the layers can slide over each other, making it useful as a lubricant and in pencils. Silicon dioxide, commonly found in the form of quartz, consists of silicon and oxygen atoms bonded together in a network structure, which also contributes to its high melting point and hardness.

In contrast, carbon dioxide is a molecular compound composed of discrete molecules held together by weaker van der Waals forces rather than covalent lattices. These molecules consist of one carbon atom covalently bonded to two oxygen atoms, but they do not form a continuous network structure. Therefore, carbon dioxide cannot be classified as a giant covalent substance because its structure does not have the extensive bonding network characteristic of the other options.