Quite how big can a molecule get?

[the tree]

For example, a very basic hydrocarbon. [math]{H}_{n}{C}_{2n+2}[/math] So when n = 1 you have methane and when n = 2 you have ethane et cetera.

 

What if n was equal to a few million, billion even? Would the molecule eventually just fall apart and have to find something less big to bond with? Or could n keep on getting bigger indefinately?

[woelen]

For example' date=' a very basic hydrocarbon. [math']{H}_{n}{C}_{2n+2}[/math] So when n = 1 you have methane and when n = 2 you have ethane et cetera.

 

What if n was equal to a few million, billion even? Would the molecule eventually just fall apart and have to find something less big to bond with? Or could n keep on getting bigger indefinately?

In fact, there is no real upper limit on the size of n. In practice, however, it will be more and more difficult to create a pure compound. For very large n you will get mixtures, which are solids.

 

Another example of a huge molecule is graphite. It is a layered structure of carbon atoms, arranged at the vertices of hexagons. Think of an infinite plane of benzene rings, where the edges of the rings are shared with the neighbouring rings, just as in naphtalene.

 

Diamond also is a macro molecule, consisting of carbon atoms, bonded to each other in a 3D structure (as far as I remember, each carbon is bonded to 4 other carbons at the vertices of a tetrahedron).

 

In fact many compounds can be regarded as single large molecules. In practice, there are defects and the molecules are not really macro, but molecules of millions of atoms are quite common. In very pure crystalline solids, the number of atoms, fitting in the ideal pattern can easily exceed billions.

[Yggdrasil]

Polysaccharides such as cellulose, amylose, amylopectin, and glycogen are other very large molecules. For example, glycogen can consist of up to about 120,000 units of glucose, so this gives a molecular weight of about 19 million.

 

Another example of a large molecule is DNA. Each chromosome consists of two molecules each forming a strand of DNA. For example, human chromosome 1 consists of about 2.5x10⁸ base pairs. Polypeptide chains can consist of up to 27,000 amino acids (approx molecular weight of 3 million). A strand of hair could even be considered a single molecule since the collagen fibers in hair are all interconnected by interchain disulfide bonds.

 

Of course these sizes aren't that big compared to the covalently networked solids described by woelen.

[latentheat]

I think paraffin wax(candle wax) is a mixture of long chain alkanes with n > 20.

 

Also, the protein called enaptin in nerve synapses is an enourmous polypeptide. It's formula is C₄₄₁₈₉H₇₁₂₅₂N₁₂₄₂₈O₁₄₀₀₇S₃₂₁

[Borek]

Polymers can get quite large. Especially these creating interchain bonds - bakelite behaves this way if I recall correctly. Looks like whole casing of my gradpa radio was one particle

 

Best,

Borek

--

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[mmalluck]

Think about the silicon ingots that are cut up into waffers for computer chip production.

 

Each ingot is a single crystal with purities measured in realm of less than one part contanimant per billion atoms.

 

That makes for one huge molecule!

 

Make your own silicon with this webpage

[EL]

For example' date=' a very basic hydrocarbon. [math']{H}_{n}{C}_{2n+2}[/math] So when n = 1 you have methane and when n = 2 you have ethane et cetera.

 

What if n was equal to a few million, billion even? Would the molecule eventually just fall apart and have to find something less big to bond with? Or could n keep on getting bigger indefinately?

 

Although I agree with the correct information that was posted in reply to your tricky question, when one attempts to answer a question such as "What is the molecular weight of graphite or diamond?" the reasonable answer is that the question is a wrong question. The modern atomic theory tells us about elements and their valence. As my friends explained to you, polymers and crystals do have an infinite extent of its structural periodicity. But Sodium Chloride too comes in crystalline form yet its SMALLEST unit of identity is composed of two atoms bound ionically, it is THAT _smallest_ unit of the substance that we call a molecule, hence in case of pure elemental substances the molecule is atomic. Gasses have a much clearer concept of molecules, and that is because of the diffuse nature of gases. When it comes to proteins we have primary chains bent into secondary planes folded into tertiary structures; another path is twisting a chain into a secondary helix that could bend into a tertiary and folded into a quaternary structure as well. Since amino acid sequences are reportedly a code, then we do not expect periodical repetitions to exist meaningfully, and the unit molecule could be a chromosome or an organelle, but we do not treat them as molecules. They are biochemicals with a level of complexity that exceeds the equivalently weighted pure materials.

Polymers, have periodical molecules and macro-molecules, which are evident from the name and the macro-molecular weight. Poly ethylene, Poly carbonate, Poly propylene, Poly Urethane, etcetera, all have a name after the word "Poly" indicating the name of the periodical molecule, which is the smallest unit of identity, but the macro-molecule is physically disconnected from similar macro-molecules and they are identified by the degree of polymerisation.

To sum this issue up, we have macro-molecules that can grow until you run out of the substrate micro-molecules that are being bonded to the macro-structure. Of course there are stereogeometrical restrictions, such as when the active end of the macro-molecule is screened or terminated or even hidden inside the coiled chain, and so on.

Some polymers extend in two directions linearly, and others extend in two dimensions forming membranes.

Then we may look at the problem in terms of identity, which holds the properties of the substance.

This brings us to the point of classifying molecules into micro-molecules, which are always members of a homogenous substance and macro-molecules that could be homogenous polymers or heterogeneous aggregates of matter. Take for example RNA, which means ribonucleic-acid, it took its name due to the identification of sugar-ribose molecules and nucleic acids, which are a limited subset of amino acids, and then there is the phosphate, which is not even included in the name. We identify them as constituent molecules within the heterogeneous-macro-molecule.

 

Therefore, the issue was never how big, but rather how small; we always look for the smallest group of atoms that identify the molecular structure and not the biggest, the properties of which only the molecular weight is of great significance.

[YT2095]

there isn`t "technicaly" an upper limit, esp when you consider polymers

[DQW]

This is really a question of nomenclature. For instance, a crytalline solid is not considered a molecule.

[akcapr]

polymers can just go on for infinite lengths

[Bluenoise]

Yeah if you took the haploid amount of your dna attached the chromosomes length to length and held it out at arms lenth it would just about touch the ground.

When you get to seriously long molecules the problem of getting a large one become a mechanical problem not a chemical one.