Advanced Placement® Standardized Tests
This is a phrase that many students may encounter during high school. For some, it might be that one time your advisor gave you the option to learn more about AP® classes; for others it could mean months of studying, tutoring, and preparing for tests. No matter what the scenario is, there is one AP® Chemistry topic that is much more interesting than it sounds, and it’s worth exploring.
Today, I want to discuss crystalline solids and their lattice structures. Your chemistry textbook (or the Internet and library) has an abundance of information about this, though for our sake I am only going for the elevator pitch.
Almost all of the solids or substances you might think of as a solid (I’m not talking about you, Jell-O®) are considered crystalline solids. This means that under conditions when the energy or temperature is low enough, many atoms or molecules will arrange themselves into a repeating, 3-D pattern known as a lattice structure. To better understand what an actual lattice structure is, let’s use a real life example.
To the left here, we see some construction workers on a building site. What isn’t seen is that these workers could be hundreds, if not over a thousand, feet above the ground. To be able to achieve extreme stability and resilience, the workers are taking advantage of a crystal lattice structure.
The steel, aluminum, or glass fiber rods are exerting what is known as a “normal force” that opposes the force due to gravity. As the weight of the scaffolding, equipment, and workers increases, the force that the scaffolding is exerting in the upward direction increases. You can almost imagine this as a bunch of heavy, identical storage boxes stacked on top of one another
How does this compare to atoms and molecules? Of course it gets a little more complex with forces of attraction coming into and out of the crystal in more than one or two directions, but these are just some of the fundamentals of a lattice structure. Any structure that is favored when intermolecular energy is at a minimum is considered a lattice structure. You are observing a simple cubic structure*, although there are many possible orientations for the unit cell.
In modern research, many chemists, physicists, and engineers look at atomic and molecular structures that are formed under a variety of energy levels. For some advanced reading, you can look for material on infrared spectroscopy, a method by which scientists use energy pulses to probe molecular structure and bonding.
*Fun Fact: Polonium (Po, element 84) is the only known-to-man element that exhibits a simple cubic structure and was discovered by Marie Curie and her husband Pierre in 1898.
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Photo Credit: Daniele Levis Pelusi & Hush Naidoo Jade Photography