Final answer:
A covalent network crystal is made of atoms bonded covalently in a rigid, 3D network resulting in a hard material with high melting points that does not conduct electricity, unlike ionic crystals which can conduct electricity when molten or in solution.
Step-by-step explanation:
A covalent network crystal differs from an ionic crystal in several key ways. The primary distinction is that a covalent network crystal consists of a three-dimensional network of atoms connected by covalent bonds, resulting in a structure that is hard, brittle, and has extremely high melting and boiling points. These crystals, such as diamond and quartz, do not conduct electricity because they are composed of atoms rather than ions.
In contrast, ionic crystals are composed of alternating positive and negative ions. These crystals conduct electricity when molten or dissolved in water because the ions are free to move. Ionic crystals typically have high melting points as well, but they are generally more brittle than network covalent crystals.
Therefore, when comparing the two, both can have high melting points, and both have a form of atomic order. However, the correct option from your choices that distinguishes a covalent network crystal from an ionic crystal is that only one of them is a poor electrical conductor, which applies to the covalent network crystal.