For example, I know there are polymers with thousands (millions?) of carbon atoms chained together.
How about a Bose-Einstein concentrate - could that be considered a molecule, or is it just a bunch of atoms in the same quantum state?
Is it theoretically possible to have a single molecule that’s so large, you could see it with the unaided naked eye?
Somewhat related question: in a bowl of table salt, the crystals of NaCl seem approximately the same size. Since Na & Cl form a regular lattice, why don’t all the moleculrs join together and form one large crystal? Why is the crystal size so regular? Something to do with electrostatic or van der Waals forces?
Not only is it theoretically possible, it is commonplace. Almost any rigid PVC object you find is just one molecule, or at most a handful of molecules. The production process is such that every polymer chain is cross-linked to every other chain, producing what is in reality one huge molecule. Something as large as a PVC deckchair fo example is just one molecule, and of course more than large enough to see.
Entirely to do with seives and production processes. Sea salt as it is produced at the site consists of crystals of all sizes. This is later dissolved again, refined and processed for sale. Some of the procesing produces the large crystals seen in stock feed and kosher salt. Some of it produces the table salt size crystals. There is no physical reaspn for the size of table salt crystals, it’s entirely due to how they are processed.
Right idea, wrong example. PVC polymers are of moderate molecular weight (10,000 - 100,000, e.g. 200-2000 atoms). They are not cross-linked.
Thermosetting resins are cross-linked (e.g. oil paints, epoxies, fibre glass resins, stoving enamels). It is an interesting question whether even in these one ever gets to one chair-one molecule stage. I seem to recall that it may not due to cyclisation events and domain effects, but the molecules remain very large even so.
Whether one views a salt crystal as a single molecule is a matter of definition. Most would say not as a molecule is generally regarded as needing covalent linkages, rather than ionic (salt) linkages.
The best example of a large molecule that I can think of would be a diamond. It’s just carbon linked covalently in a huge pyramidal lattice.
Astronomers have just identified an enormous diamond (which they’re calling “Lucy”) somewhere in the night sky. It’s about 1/10 the size of the earth. So molecules can get pretty big!
Well, since the largest molecules would be proteins, I would go by what it says here, at the Protein Data Bank. It looks like the biggest molecule would be beta-galactosidase, with about 132 thousand atoms forming the molecule. The second largest is chaperonin with 116 thousand atoms. But, don’t get too excited. As it says here, none of these huge molecules are big enough to seen (even with a microscope).
Also, I don’t think diamonds would qualify as single molecules, since the generally accepted definition of “molcule” (for a Chemist at least) is:
Of course now we can start arguing about how many carbon atoms you need to have to get the chemical properties of a diamond. but i think it would definitely be less than 132 thousand.
Also, by this definition, polymers and copolymers don’t count as single molecules, since you can split them, and they still show the same chemical properties.
Well, since the largest molecules would be proteins, I would go by what it says here, at the Protein Data Bank. It looks like the biggest molecule would be beta-galactosidase, with about 132 thousand atoms forming the molecule. The second largest is chaperonin with 116 thousand atoms. But, don’t get too excited. As it says here, none of these huge molecules are big enough to seen (even with a microscope).
Also, I don’t think diamonds would qualify as single molecules, since the generally accepted definition of “molcule” (for a Chemist at least) is:
Of course now we can start arguing about how many carbon atoms you need to have to get the chemical properties of a diamond. but i think it would definitely be less than 132 thousand.
Also, by this definition, polymers and copolymers don’t count as single molecules, since you can split them, and they still show the same chemical properties.
If you look here, there is a simple do it yourself method for extracting DNA from anything living (like spinach, peas, onions). I don’t recommend trying this with human subjects. You’d need a really big blender. Anyway, at the end you should get visible strands of DNA, so I guess maybe DNA would count as the largest molecule, but then again, it would probably count as a type of polymer or copolymer (sure the genetic coding is different or destroyed if we split the DNA, but the chemical properties are still the same), so I go back to my original choice of beta-galactosidase.
the threads of DNA visible to the naked eye that you can extract from many sources is not one molecule, rather tens of trillions of much smaller strands clumped together. The can be readilly disaggregated into a soluble solution.
While we’re on the subject, and not to hijack this thread, but can any of you explain the following to me, bearing in mind that I have only the most basic understanding of thing atomic and molecular?
I’ve been given to understand that if the nucleus of an atom were the size of a grain of sand and situated in the middle of a football field, that the electrons orbiting it would be as far out as the end zones. I realize there is probably some argument to be made about this as I imagine electron orbits vary with the type of atom involved, but my question is: How is solid matter created by a group of atoms given that the atoms themselves are only small bits that are separated by large empty spaces. Where is the solidity and what causes it. And what keeps nucleuses and electrons from running into each other and screwing things up?
Any insight you could contribute would be much appreciated.
Thanks for the intelligent replies everyone. It sounds like using the strict definition, the largest true molecule would be a protein, and even that is nowhere near visible to the naked eye. Now we just need that tunneling quark microscope so we can probe atoms
So there’s no reason a single crystal of table salt couldn’t be, say, a cube of side length 6 feet?
Its all about forces. some attract, some repel. Some have long range, some have short range. Increase distance between nuclear/atomic/molecular entities and the forces of attraction take over. Decrease distances between nuclear/atomic/molecular entities and the repulsive forces increasingly dominate. Solid objects as we perceive them are merely the manifestation of nuclear/atomic/molecular forces in equilibrium.
The perception of solidity has everything to do with the electromagnetism. Electrons theoretically take up about zero space and weigh very little. But they’re moving very fast around and between nuclei, and effectively “screen” them. These electrons can scatter light (giving something the appearance of being opaque) and, when they’re forced close together, they strongly repel one another. On or near the surface of an object, there’s plenty of positive charge “below”, but none “above” to create a symmetrically neutral environment on super-atomic scales. So, when you get your hand next to a wall, all that unshielded negative electric charge smeared out over both surfaces imparts a strong repulsive force.
Electrons and other subatomic particles obey the laws of quantum mechanics, which only allows electrons to exist at certain distances from the nucleus. Electrons don’t crash into the nucleus because there is no allowed quantum state that would bring them close enough. An electron’s state is represented mathematically by the variables n, l, m[sub]l[/sub], and m[sub]s[/sub]. Basically, the greater the value of n, the farther the electron is from the nucleus, and there is no value of n for which that distance is zero.
Now to explain solidity. Enter the Pauli Exclusion Principle. It states that no two electrons can have the same values of all 4 quantum numbers (n, l, m[sub]l[/sub], m[sub]s[/sub]) simultaneously and therefore cannot exist in the same place at the same time. Because of this principle, electrons are forced to pile up around the nucleus in shells so that a few (typically 2 or 8) are on the outside. When atoms get too close together, the electrons in each atom’s outermost shell repel each other. So the “solidity” you feel when, say, pressing the keys on your keyboard, is actually a combination of the Pauli exclusion principle and the electromagnetic repulsion of outer electrons in your fingertips and keys, supported by the range of allowed quantum states an electron may inhabit.
M. Gilbert and J.C. García-Quesada. Chemical crosslinking of rigid PVC. Plastic, Rubber and Composites. Processing and Applications. Vol. 28, Nº 3, pp 125 (1999). http://iq.ua.es/~jcgq/publici.html
Well, could you please give me an example of one? And 216 cubic feet of rock salt don’t count, because a molecule should have covalent bonds and not ionic bonds. In a similiar vein, diamond crystals don’t count, and polymers and copolymers also don’t count.
Here is a pretty exact definition of “molecule”:
A molecule is a combination of two or more atoms that are held together by covalent bonds. A molecule is the smallest unit of a compound that still displays the properties associated with that compound. Molecules may contain two atoms of the same element, such as O[sub]2[/sub] and H[sub]2[/sub], or they may consist of two or more different atoms, such as CCl[sub]4[/sub] and H[sub]2[/sub]O. In the study of chemistry, molecules are usually discussed in terms of their molecular weights and moles.
Ionic compounds, such as NaCl and KBr, do not form true molecules. In their solid state, these substances form a three-dimensional array of charged particles. In such a case, molecular weight has no meaning, so the term formula weight is used instead.
And here is the official last word from the IUPAC (International Union of Pure and Applied Chemistry):
So, in other words, we are looking for something that has a well defined molecular weight, so we can say that we have one mole of whatever molecule we are using. If you say that a bottle made of PVC represents a single molecule, does that mean we have to collect 6.022 X 10[sup]23[/sup] bottles to have one mole of PVC? Doesn’t make sense does it? On the other hand, you can definitely say that we need 6.022 x 10[sup]23[/sup] molecules of beta-galactosidase to have one mole.
Now name a molecule larger than that using the official IUPAC definition.
well you can in principle cross-link anything (including ones brain if left under an ioniser long enough), but 99% of PVC products are not cross-linked. Same is true of 95% of polyethylene products.
Anyway, at the end you should get visible strands of DNA, so I guess maybe DNA would count as the largest molecule, but then again, it would probably count as a type of polymer or copolymer (sure the genetic coding is different or destroyed if we split the DNA, but the chemical properties are still the same), so I go back to my original choice of beta-galactosidase.