Crystalline Solids: Videos & Practice Problems

Crystalline solids are characterized by their highly ordered and symmetrical patterns, which are made up of repeating structures called unit cells. These unit cells are the smallest portion of the crystal lattice, which is an arrangement of identical unit cells in three-dimensional space. Lattice points are specific areas within the crystal lattice occupied by atoms with identical surroundings. When examining the structure of crystalline solids, it's important to consider the coordination number, which is the number of neighboring atoms surrounding a single atom in the crystal lattice, and the packing efficiency, which is the proportion of space in a unit cell that is occupied by atoms. As the complexity of the unit cell increases, so does the coordination number and packing efficiency. This is evident in the three common types of cubic unit cells, where the trend shows that more complex structures have higher coordination numbers and greater packing efficiency.

Crystalline Solids Structure

Now, when we use the term crystalline solids, just realize that crystalline solids are closely packed structures arranged in a highly ordered and symmetrical pattern, and with them there are three key terms you need to keep in mind. First, a unit cell. We're going to say a unit cell is the smallest and most fundamental repeating structure of a crystal lattice, and a crystal lattice itself represents identical unit cells arranged in a three-dimensional space.

And with that we have lattice points which are just areas within a crystal lattice which is occupied by an atom with identical surroundings. So here, if we take a look at a crystalline solid, we're going to say that the entire structure here, which looks like 8 cubes stacked with each other, this whole entire structure represents our crystal lattice. Our unit saw is one of those individual cubes.

So here this would be our unit cell and the edges and points of each of those cells. That is our lattice point. So just realize when you're looking at a crystalline solid, the crystalline solid can be seen as the macro view of it. And then we're going towards the lattice point, which is the micro the smallest portion of a unit cell. So just keep these terms in mind when looking at any type of crystalline solid.

Crystalline Solids Example

What is the most basic repeating structure of a crystal lattice? So remember a crystal lattice we can be seen as a crystalline solid, which is the macro view of it. If we're talking about the repeating structures that comprise A crystalline solid, that's just our unit cell. So here our option would be answer B.

Remember, the crystalline solid is the macro view. The lattice point is just looking at one of the edges or quarters of our unit cell and then realize here that our lattice point is involved with the atom itself, right? And again, our best option will be option B.

Cubic Unit Cells

Hey everyone. So when it comes to the cubic unit cell, realize that there are three types of unit cell arrangements and they each possess different coordination numbers and packing efficiencies. Now when we say coordination number, this is just the number of neighboring atoms surrounding one atom in a crystal lattice.

Here if we were to take a view of a crystal lattice, let's have this as our atom of reference. That atom we're going to say because of this image, the coordination is equal to 6. We're going to say every atom in the crystal lattice is surrounded by 6 atoms themselves. When we say packing efficiency, we're going to say packing efficiency is the percentage of occupied atoms by occupied space by atoms in a unit cell.

Now we're going to say the trend is the greater the complexity, then the larger the coordination number will get. And as a result of this, we're going to have a larger packing efficiency. Here we have an image of three typical types of cubic unit cells. And as we go from left to right, we can see that the cubes themselves get filled in more and more.

They're becoming more and more complex with these blue regions representing additional atoms that can be comprised within a particular unit cell. So just remember, as we're heading from left to right, we have an increase in complexity which will result in larger coordination numbers greater than 6 and that would also result in a larger packing efficiency.

Here’s what students ask on this topic:

What is the difference between crystalline and amorphous solids?

Crystalline solids and amorphous solids differ primarily in the arrangement of their constituent particles and their long-range order.

Crystalline solids have a highly ordered structure, where the atoms, ions, or molecules are arranged in a repeating pattern extending in all three spatial dimensions. This regular arrangement results in a well-defined, sharp melting point, because the crystalline structure breaks down at a specific temperature. Crystalline solids also tend to have flat surfaces or faces that intersect at characteristic angles, a feature known as crystal faces. Examples of crystalline solids include table salt (sodium chloride), diamonds, and quartz.

Amorphous solids, on the other hand, lack this long-range order. Their constituent particles are arranged more randomly and do not form a repeating pattern. As a result, amorphous solids do not have a sharp melting point; they soften over a range of temperatures when heated. They also do not exhibit flat crystal faces like crystalline solids do. Common examples of amorphous solids include glass, plastics, and gels.

In summary, the key difference lies in the particle arrangement and the resulting physical properties such as melting points and the presence of crystal faces.

What are crystalline solids?

Crystalline solids are materials whose constituent atoms, ions, or molecules are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. This orderly pattern of arrangement is repeated at regular intervals, giving the solid a definite shape and structure. The regularity of the crystal lattice results in unique and characteristic physical properties such as cleavage planes, where the solid can be split smoothly along certain lines, and anisotropy, where the properties of the material vary depending on the direction of measurement.

In a crystalline solid, the precise arrangement of the particles also leads to sharp melting points, as the crystal structure breaks down at a specific temperature. Additionally, these solids often have flat surfaces, or faces, with specific, characteristic angles between them. Examples of crystalline solids include table salt (sodium chloride), diamonds, and metals. The regular internal structure of crystalline solids can be determined by techniques such as X-ray diffraction.