Now, the most fundamental distinction between solids is that they can be classified as either crystalline or amorphous solids. When we say crystalline solid, we're referring to atoms, ions, or molecules that have highly ordered arrangements throughout. And amorphous solids are particles that are randomly arranged throughout with no discernible pattern. Crystalline solids include ionic solids, molecular solids, covalent network solids, and metals.
For ionic solids, the smallest unit is the ionic part because its electrostatic forces are the attractions between a positive ion and a negative ion. We know the definition of ionic compounds as a cation connected to an anion. That cation could be a metal or an ammonium ion, and the anion is typically some non-metals. Ionic solids tend to be brittle and yet hard with high melting points. A good example is solid sodium chloride for our ionic compound.
Molecular solids have molecules as their smallest unit. Their electrostatic forces are the intermolecular forces. They tend to have soft textures and low to moderate melting points. A good example of a molecular solid is ice, which is just H2O solid.
Covalent network solids have atoms as their smallest units, and their electrostatic force comes from covalent bonds. If you want to understand the hardest materials in nature, you're referring to covalent network solids. They have very hard textures and very high melting points, even higher than ionic solids. A great example and one of the hardest natural substances is diamond; remember, diamonds are just a form of carbon. But carbon can also take other forms in nature, such as graphite.
Now, metals are composed of metal atoms, and we're dealing with metallic bonds, which involve the pooling of electrons on the surface of the metal. Metals can be lustrous, meaning they are shiny, and their textures can range from soft, like sodium, to hard, like titanium. Metals also have high melting points.
Finally, we have amorphous solids, which differ from all these crystalline solids. They are composed of atoms, ions, molecules, or polymers, which often refer to different forms of plastics. Because they don't have a discernible pattern, they somewhat resemble the crystalline solids we mentioned above. Because of their unusual nature and composition, they tend not to have a distinct melting point, and what is really weird about them is their ability to flow, a trait typically reserved for liquids and gases. A great example here is glass or tar, the material that we use on roads. Tar is able to flow, resembling black sticky stuff.
So, these are the different types of crystalline solids and how they differ from an amorphous solid.
