Do not confuse unit cells for crystalline solids with molecules. Molecules are discrete entities. They are complete building blocks in and of themselves. A molecule of sugar is a molecule of sugar whether it is in a "crystal" solid with other molecules or not. The chemical bonds that form between atoms in the molecule use up the available covalent bonding capacity of the atoms. What attracts molecules to molecules are the much weak forces due to concentrations of charges within the molecules, whether they are temporary and due to induced dipole charges, or permanent dipoles due to polarity in the covalent bonds within the molecule.
Crystalline solids don't have molecules. These are ionic solids which are held together by localized charges on the atoms or subgroups of atoms because of uneven numbers of protons and electrons inside the group. Each ion attracts other nearby ions, and those attract still more. We can't pull the atoms in a unit cell out without disturbing other unit cells. The atoms in a unit cell are linked to other unit cells directly. Like a checkerboard, there is no "end" to the pattern until we run out of atoms to fill it.
The criteria for identifying a unit cell are that it is symmetrical and infinitely repeatable. It is possible that more than one unit cell description will provide a good description of the organization of atoms within the lattice. If we come up with two that give the same ratio of atoms, we can chose to use whichever one best suits our current purpose. But if two unit cell descriptions appear to account for the locations of the atoms inside but give us different formulae for the substance, we've done something wrong.
You'll notice above that an atom may not lie entirely inside its unit cell, and that it can belong to more than one cell. Look at what happens in three dimensions, depending on where the atom is with respect to the boundaries of the cell (the surfaces, edges, and corners of the cell).
Now let's look at what happens when we put unit cells together in three dimensions.
Let's apply this to cuprite, Cu2O.
Check out the Online Chemistry Text and LbreTexts on Unit Cells.
© 2005 - 2021 This course is offered through Scholars Online, a non-profit organization supporting classical Christian education through online courses. Permission to copy course content (lessons and labs) for personal study is granted to students currently or formerly enrolled in the course through Scholars Online. Reproduction for any other purpose, without the express written consent of the author, is prohibited.