Which of the following properties are typical characteristics of a covalent- network solid, a metallic solid, or both: (a) ductility, (b) hardness, (c) high melting point?

Ductility is the ability of a material to be drawn into a wire or deformed without breaking. This is a property of metallic solids due to their "sea" of valence electrons that allows the metal atoms to slide past each other when a force is applied. Covalent-network solids are hard and brittle and do not exhibit ductility. So, ductility is a typical characteristic of a metallic solid.

Hardness is the resistance of a material to deformation or penetration. Covalent-network solids are hard due to their strong covalent bonds. These solids are also brittle, meaning they break rather than deform under stress. Metallic solids can vary in hardness but are generally more resistant to deformation because of their crystalline structure. So, hardness is a typical characteristic of a covalent-network solid.

High melting points occur when a solid requires a large amount of energy to overcome the interactions between the atoms and change from a solid to a liquid phase. Covalent-network solids have high melting points since it takes a large amount of energy to break the strong covalent bonds within the network. Metallic solids can also have high melting points due to their crystalline structure and bonding strength. However, not all metallic solids have high melting points. For example, mercury remains a liquid at room temperature, and some metals may melt at low temperatures. Therefore, a high melting point is a typical characteristic of both covalent-network and metallic solids but may not always be observed in metallic solids.