What is the difference between a molecule and a crystal

What is the difference between a molecule and a crystal or unit cell of a crystal? Is each unit cell of a given crystal one molecule, or is the whole crystal of whatever particular size one molecule? Is a crystal that particular kind of molecule that is infinite (would go on forever if not stopped by the usual conditions), unlike the other kind which has a definite end to what can be in it?

Calling something a crystal describes the structure of the atoms, ions, or molecules in the substance, the regularly repeating three dimensional pattern that they are arranged in. The unit cell is made up of the individual atoms or molecules.

One way to look at the difference is the bonds between the atoms. Strong (covalent) bonds between atoms form molecules. Week (non-covalent) bonds between (atoms in) molecules form crystals (or liquids, other solids, etc). Crystals can be pretty strong but the bonds between the molecules are still less strong than those between atoms in a molecule. Wait…

I get it. Diamond which isn’t made of complex molecules, but of atoms of carbon bonding in a regular latice. Good question. Are the bonds between carbon atoms different than the single bonds between carbons in a saturated fatty acid molecule?

Is that the question? Do you understand the difference between the NaCl molecules in a salt crystal and the whole crystal?


I don’t. Could you expand on this a little?

I assume your being Tongue in cheek, DS

IIRC there is no difference, NaCl, i.e. 1 Na, 1 Cl just reflects the (aprox.) ratio of atoms within the crystal’s structure.

Not at all. I’m pretty ignorant in chemistry and I can prove it.

Ok, basically some crystals are molecules, but not all and some molecules are crystals (well, duh!) but againn not all

To expand on what has already been said a bit: a crystal is defined as a repeating lattice of atoms. A molecule is a group of atoms chemically bonded together. A crystal may be chemically bound, or it may not – the symmetry is what defines a crystal, not the bonding.

Also, a crystal usually implies a certain scale, much much larger than the spacing between atoms. Five hundred carbon atoms in a diamond-like arrangement is called a cluster or a nanoparticle. It’s not called a C[sub]500[/sub] molecule. That would give the false impression that having 500 atoms is important to the properties of the system, when in fact a system of 501 atoms would be basically the same.

Finally, I should note that there are molecular crystals, or lattices of molecules.

Just to elaborate: NaCl (molecule) is one Na (atom) covalently bonded to one Cl (atom). NaCl crystal is many NaCl moleules non-covalently bonded: NaCl-NaCl-NaCl-NaCl… in 3 dimensions. So the crystal is just solid NaCl, but in a regular lattice structure.

In diamond (or graphite) the unit is not a “complex” molecule, but a single atom, C. My question above was, if in this case, the bonds are covalent. Then a diamond might be considered a molecule. (My ignorance)


No. NaCl does not form covalent bonds, and there is no such thing as an NaCl “molecule.” Na and Cl ions combine and recombine pretty freely in solution, and crystallize into large frameworks spontaneously when brought out of solution. It’s all ionic (opposite electric charges, and so on).

Most (but not all) molecules are formed by covalent bonds, which are stronger (electron-sharing), but when molecules are in a crystal, they are joined to other the molecules by ionic bonds, hydrogen bonds, or even van der Waals forces. It’s the orderly arrangement of atoms or molecules in a large repeating structure that makes a crystal.

The unit cell of a crystal might be the molecule; it might be a particular arrangement of molecules (the same pattern of four, say, repeated over and over); it might be an atom, or an arrangement of different atoms. The unit cell is a thoretical structure that contains the atoms or molecules, in the arrangement that repeats in the crystal, plus the space between the atoms or molecules, in such a way that the unit cells can just be plopped together to form a crystal.

This PDF explains crystal structure fairly well.

Ok. So are the carbon atoms in diamond covalently bonded?


Yes. Each carbon is located at the center of a regular tetrahedron and is bonded to four other such carbons, forming a rather lovely pattern.

A molecule is the smallest subdivision of a substance that still retains its chemical and physical properties.

If you take a salt crystal, NA-CL-NA-CL-NA-CL… you can cleave it and you still have a salt crystal. It still dissolves in water, still flavors pasta well, etc.

When you get all the way down to two units of salt, that is, NA-CL-NA-CL, you can divide it one more time into NA-CL. That is a single salt molecule.

A single structural unit of salt (one atom of each) might retain salt’s chemical properties, but it most certainly would not retain its physical properties. Nor is there any particular size where you could say the properties change: As you get too close to a single structural unit, you’ll gradually change properties. If, then, one takes that as one’s definition of a molecule, then there is no such thing as a molecule of salt (nor, for that matter, of diamond).

Do 4-dimensional crystals retain their properties when atoms are separated by 13 years on the t-axis?

A crystal is a solid whose molecular structure gives it constancy in its three-dimensional structure (inter-facial angles among others) regardless of size (down to a single molecule) or form (even if it was broken or rounded). Paraffin wax, nor glass cannot have such a property but both can be manually shaped into crystalline forms. They, however, are not true crystals.

L. No workee

Maybe it workeed 13 years ago on the t-axis?

An example of a crystal that has unit cells that are molecules (internally bonded covalently) but whose unit cells are not covalently bonded to each other is a snowflake. Ice crystals are bonded to each other via (non-covalent) hydrogen bonds.

Nitpick: Isn’t covalent bonding part of the fundamental definition of a molecule?