What’s a solid?

“Solid” refers to the state of matter in which the particles are locked into place without much freedom of movement.  They can be locked into crystal lattices or just kind of stuck together with intermolecular forces so tightly that they can’t really move around.  As we’ll see, it depends on the type of solid we’re talking about.

Solids differ from liquids in that the particles in liquids, while still stuck together, do have some freedom of motion.  Solids differ from gases in that gas molecules really don’t interact with each other much, flying all over the place.

What are the different types of solids?

Ionic solids:  Ionic solids are solids in which anions and cations (negatively and positively charged atoms or groups of atoms, respectively) stick together via “electrostatic attraction.”  By “electrostatic attraction”,  I basically mean that the opposite charges just like to stick to each other.  When they do this, they form great big crystals in which each ion is surrounded by ions with the opposite charge.  Such crystal lattices (as they’re called) are really stable, requiring lots of energy (called the “lattice energy”) to pull apart.  Examples of ionic solids include sodium chloride (table salt) and Epsom salts (magnesium sulfate heptahydrate).  For more info, click here.

Metallic solids:  Metallic solids are solids in which the positively charged nuclei are held together by a bunch of valence electrons that kind of bind the whole mess together.  These electrons are referred to as being “delocalized” because they don’t stay between two atoms as they do in covalent bonds – instead, they travel throughout the solid.  This allows the atoms in solids to move around, as the bonding electrons can move around with them.  A phrase commonly used to describe metallic bonding is “electron sea theory”, which describes the positive metal nuclei as floating around in an ocean of negative electrons that hold them together.  Just about anything you know of that’s a metal does this kind of bonding.

Network atomic solids:  Network atomic solids are great big crystals in which all of the atoms are stuck together using covalent bonds.  Because the atoms are all locked into place, these solids usually have properties very similar to that of ionic compounds (high melting and boiling point, hard, brittle, and so forth) with the exception that they don’t conduct electricity if you melt them.  Typically, gemstones (such as amethyst, diamond, and ruby) are network atomic solids.

Molecular solids:  Molecular solids occur when covalent molecules are held together by intermolecular forces.  In this type of bonding, which occurs in ice, the intermolecular forces between the molecules are strong enough to keep the molecules locked into place.  Typically, these types of solids have much lower melting and boiling points than metallic, network atomic, or ionic solids, because the intermolecular forces holding the molecules together are much weaker than those of the bonds in the other compounds.  My tutorial on covalent compounds deals mostly with the properties and behavior of this type of solid.

Atomic solids:  Atomic solids occur when noble gases are cooled to really low temperatures and lock themselves in place using very weak London dispersion forces.  You won’t run into these solids in the “real world”, because you need temperatures that are ridiculously low to see them.

Amorphous solids:  Amorphous solids, unlike the rest of these solids, have no particular crystal structure.  In an amorphous solid, the particles are just kind of stuck all over the place, with no regular bonding pattern.  Some amorphous solids are soft and rubbery (such as plastic and rubber) because they consist of long molecules which are just kind of tangled together and bound with intermolecular forces.  Other molecular solids (called glassy solids) are a lot more like network atomic solids because they consist of atoms stuck together in an irregular fashion using covalent bonds.  It won’t be much of a surprise to find that glass is an example of such a solid!