I hope you’ve learned by now that there are three states of matter that chemists worry about: Solids, liquids, and gases.  If you haven’t yet heard about these, I strongly encourage you to slap your chemistry instructor upside the head at your earliest convenience.

In any case, we’ve talked about liquids and gases in other tutorials, so let’s talk about solids.

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What’s a solid?

Solids are materials in which the atoms or molecules are locked in one place and can’t move around.  The force that holds these things together consists of either covalent bonds, metallic bonds, intermolecular forces, or the electrostatic attraction between ions.  Whichever is the case, those particles are staying put.¹

In liquids, the particles stay close to each other, but enough energy has been added that they can kind of slide around from place to place.  In gases the particles have enough energy that they don’t spend any time at all near each other, preferring to zip around everywhere.

We can classify solids based on the structure they’re held together with at a molecular/atomic level. Though the particles in all types of solid stay in one place relative to each other, the means by which they’re stuck in place can be very different.

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Crystalline solids

Crystalline solids make up one class of solids.  In a crystalline solid, the particles are all locked rigidly in place in a predictable and repeating way.  To give you an idea of what this looks like, check this out:

Here are some handy terms we use to describe crystalline solids:

• Crystal:  The great big lump of matter that you can carry around and look at.  If you were to see a diamond on the finger of some rich lady, you’re looking at a crystal.
• Crystal lattice:  The structure of the big lump of matter that you can carry around and look at. The crystal lattice of that rich lady’s diamond is tetrahedral, and shown in the above animation.
• Unit cell:  A unit cell is the smallest chunk of the lattice that, if repeated, can generate the whole thing.  Check it out:

Crystals are said to have different structures based on the arrangement of atoms.  There are like a zillion of these out there, including simple cubic, face-centered cubic (fcc), and body-centered cubic (bcc).  There’s also a lot more of them out there, but I’m not going to spend the rest of my life writing them down.

The properties of crystalline solids will obviously depend on the type of solid and the chemical makeup of the particles in it.  However, it’s probably safe to say that crystalline solids generally have the following properties:

• They’re hard:  The particles are locked in place, and don’t feel like moving much.
• They’re brittle:  If you hit them against something hard enough to make the particles shift, they probably won’t line up in a stable pattern and will fall apart instead.

OK… that’s actually about it for properties.  Instead of focusing on that, let’s talk about the different types of crystalline solids.

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Types of crystalline solids:

Let’s take a look at the different types of crystalline solids, and what distinguishes them from each other:

Ionic solids:

Ionic solids are solids consisting of cations and anions held together by the electrostatic forces between positively and negatively charged particles (or, as it is sometimes called, “ionic bonds.”) Examples of ionic solids consist of sodium chloride and every single other ionic compound you’ve ever talked about.

Properties of ionic solids:

• High melting points:  The “ionic bonds” are very stable and it takes lots of energy to separate them from each other.
• Hard:  Those ions are locked tightly in place due to the strength of the positive-negative interactions.
• Brittle:  If you succeed in shifting the ions, they’ll most likely wind up in a pattern where differently charged ions are near each other.  This doesn’t go over well, so the crystal breaks apart.
• The conduct electricity when dissolved in water or melted:  Electricity is caused by two things – moving ions and moving electrons.  When you dissolve or melt an ionic compound, the ions can move around.

Metallic solids:

Metallic solids consist of metal atoms that are packed in close proximity to one another. They are held together via metallic bonds, which consists of delocalized electrons moving throughout the entire metal lattice.  If that doesn’t sound familiar, maybe the words “electron sea theory” will jog your memory.

Properties of metallic solids:

• Malleable (bendy) and ductile (stretchy):  Though the metal nuclei are all locked in place by metallic bonds, the delocalized bonding that takes place allows them to shift around somewhat. As a result, metals deform instead of shattering.
• Generally high melting points:  Metallic bonds are strong and hold metal nuclei together tightly.  There are exceptions (looking at you, alkali metals), but generally you can think of them melting in the hundreds of degrees Celsius.
• They conduct electricity as solids:  Though there are no ions present, the delocalized bonding allows electrons to move throughout the metal without any problems.  As a result, you probably don’t want to lick electrical wires anytime soon.

Network atomic solids

Imagine a big molecule.  A really big molecule.  In this unimaginably huge molecule, all of the atoms are bonded to the other atoms with covalent bonds.

Basically, that’s a network atomic solid.  Unlike an ionic solid in which everything is stuck together with electrostatic attraction, network atomic solids hold things together with covalent bonds.  As a result, you end up with a crystal similar to an ionic solid, but with a few differences.  Network atomic solids include diamonds (shown above) and quartz.

Properties of network atomic solids:

• High melting points.  Just as is the case with ionic solids, there are really strong forces (i.e. covalent bonds) holding the particles in a network atomic solid together.  It takes a lot of energy to break these bonds, so if you want to melt these things you’ve got to heat them quite a bit.
• They’re hard and brittle.  Just as is the case with ionic compounds, the crystals are hard because the particles are locked tightly in place and the crystals are brittle because if you manage to put enough energy into the crystal to break it apart, you’ll destabilize the whole thing.
• They don’t conduct electricity.  As mentioned above, electricity occurs when either electrons or ions move.  Given that there are no delocalized electrons and that there are no ions, these don’t conduct no matter what you do.

Molecular solids

Think way back to when you learned about intermolecular forces.  Remember how we discussed how they described how things behave in a solution?  Well, it turns out that they are good in solids, too.  You see, a molecular solid is a crystalline solid where all of the molecules are locked in place with intermolecular forces.  Ice is a familiar example of a molecular solid, as is sugar.

Properties of molecular solids:

• Lower melting point than for network atomic or ionic solids.  While those two types of solid are held together by bonds of one kind or another, molecular solids are held together with intermolecular forces.  Because intermolecular forces are far weaker than bonds, it takes less energy to pull the molecules apart.
• They’re softer than network atomic or ionic solids.  That’s not to say that if you get hit with a chunk of ice that it’s soft, or anything, but consider the effectiveness of using a stick to scratch both a piece of ice or a diamond.  Unless you’re really dumb, you probably realize that the ice will scratch, mainly because the forces holding the molecules together are easy to overcome.

Atomic solids

Noble gases have very weak intermolecular forces, with only very lame London dispersion forces keeping them from wandering away from each other.  However, if you manage to cool a noble gas enough, even these weak forces will cause the atoms to stick to each other.  The resulting solid is an atomic solid.  There’s not really much to say about atomic solids, except that they have really low melting points.

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Amorphous solids

Aside from your crystalline solids, you also have your amorphous solids.  Since crystalline solids are defined as being solids with long-term molecular order, it’s probably not too hard to figure out that amorphous solids have no particular arrangement that they follow.  As a result, it’s pretty hard to say much about them as a whole.  Fortunately for you, I don’t mind making wild generalizations in the cause of science.

Types of amorphous solids (more or less):

• Plastics / rubbers:  Plastics consist of long molecules that are all tangled together like spaghetti. Unlike crystalline solids which are locked in place like LEGOs, plastics and rubbers just have a bunch of long tangly molecules that are sort of knotted together.  By analogy, think of a cat playing with a ball of yarn.  If you let the cat play long enough, and if you give it enough yarn, it will eventually tie itself into a little solid ball of cat/yarn/lint.  That’s a plastic.

• Glasses:  A glass is a material that’s held together by real bonds of one kind or another, but doesn’t have any long-range order.  For example, consider window glass.  It looks like a crystalline solid, but hot window glass cools so quickly that the atoms don’t have time to line up in a nice, neat pattern.  As a result, it’s more a jumbled bunch of bonded atoms rather than a nice neat crystal.  Interestingly, glasses often have many of the same characteristics of network atomic solids because they have the same formulas and the same type of bonding:  For example, window glass is the amorphous solid version of quartz.

Fun fact:  It’s often said that window glass isn’t a liquid, but is rather a solid.  The people who say this are morons, because it is, in fact, a solid.

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Footnotes:

1. Yes, I know:  The particles don’t stay put completely, but wiggle around a bit.  The key idea here is not that the particles are totally unmoving, just that they don’t move relative to the particles around them under most circumstances.

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Image credits:

• Dude getting punched: Image courtesy of David Castillo Dominici at FreeDigitalPhotos.net
• Grandma: Image courtesy of photostock at FreeDigitalPhotos.net
• Rotating diamond:  Public domain.
• Diamond unit cells:  I, RosarioVanTulpe [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY-SA 2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)%5D, via Wikimedia Commons
• Simple cubic: By DaniFeri    iThe source code of this SVG is valid.            This vector image was created with Inkscape. (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)%5D, via Wikimedia Commons
• Smiling bulb woman:  Image courtesy of imagerymajestic at FreeDigitalPhotos.net
• Ice queen:  Image courtesy of Charisma at FreeDigitalPhotos.net
• Big pile of wool:  Image courtesy of Dino De Luca at FreeDigitalPhotos.net