Now, the most fundamental distinction between solids is that they can be classified as either crystalline or amorphous solids. Now when we say crystalline solid, we're going to say here that atoms, ions, or molecules have highly ordered arrangements throughout. And amorphous solids are particles that are randomly arranged throughout, with no discernible pattern. When we refer to crystalline solids, we're talking about ionic solids, molecular solids, covalent network solids, and metals.
For ionic solids, the smallest unit is the ionic part, and that's because its electrostatic forces are the attraction between a positive ion and a negative ion. Remember, the definition of ionic compounds is a cation connected to an anion. That cation could be a metal or an ammonium ion, and the anion is just some nonmetals. Ionic solids tend to be brittle yet hard, and they have high melting points. A good example is solid sodium chloride for our ionic compound.
Now, molecular solids—their smallest unit is the molecule. Their electrostatic forces are the intermolecular forces. They have soft textures and low to moderate melting points. A good example of a molecular solid that we can think of is ice, which is H2O solid.
Covalent network solids, their smallest unit is the atoms. They use covalent bonds for their electrostatic force. 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 our ionic solids. A great example, one of the hardest natural substances in nature, is diamonds. So remember, diamonds are just a form of carbon. But that's not the only form that carbon can take within nature; it could also take the form of carbon graphite as well.
Metals, of course, are composed of metal atoms. We're going to say here that they deal with metallic bonds, and that's just the pulling of electrons on the surface of your metal. They can be lustrous, which means that they are shiny. They can vary from soft textures like sodium to hard textures like titanium. They also have high melting points. Here, an example is the ones we gave, sodium or titanium.
Finally, we have our amorphous solids, which are different from all the other crystalline solids. Here they are composed of atoms, ions, molecules, or what we refer to as polymers. When we say polymers, we're talking about different forms of plastics. Because they don't have a discernible pattern, they kind of resemble some of the crystalline solids we mentioned above. Because of their weird nature and composition, they tend not to have a distinct melting point. And what's really weird about them is that they're solids but are able to flow. So that's not a normal thing. The ability to flow is usually reserved for liquids and gases, not solids. A great example here, you could say, is glass or tar. So tar, the material that we use on roads. So that's an example of an amorphous solid. If you can understand what tar looks like, black sticky stuff, it's able to flow. So, these are the different types of crystalline solids and how they are different from an amorphous solid.
