There really is no limit to the size of molecules.
Well, if you had a molecule the size of the solar system, it would collapse under its own gravity and form a black hole- but ignore technicalities like that.
DNA strands are single molecules, and they can be several inches long, which is about a billion atom-lengths. Despite their great length, these molecules aren't much more than 10 atoms wide. DNA molecules pair up and coil into chromosomes, which can be seen with a good microscope.
Diamonds are also single molecules, which is one reason for their unusual hardness- to break a diamond, you must break trillions of atomic bonds. The world's biggest uncut diamond weighed 1 1/3 pounds, so that molecule had 30 trillion trillion carbon atoms! In diamond, each carbon atom connects to four of its neighbors, forming a pyramid.
The bonds form an interlocking network, like a 3-D version of the triangular scaffolding you see on bridges. Triangles have great structural strength, which also contributes to the hardness of diamond. Atoms are a bit like Zaks. You can build something as big as you want, as long as you have enough pieces and you follow the linking rules. Carbon (C) and silicon (Si) must have four bonds, nitrogen (N) and phosphorous (P) must have three, oxygen (O) and sulphur (S) have two, hydrogen (H) and chlorine (Cl) have one, etc.
There are exceptions to these rules, but you can use them to figure out how most molecules are put together.
Try it for simple molecules like water (H2O), carbon dioxide (CO2), methane (CH4), nitrogen (N2), cyanide (HCN), or hydrochloric acid (HCl). If you're ambitious, try it for benzene (C6H6) or alcohol (C2H5OH).
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