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.