# how Can an organic molecule Be Stronger Than metals? **URL:** \ **Category:** Factual Questions\ **Created:** [November 26, 2008, 2:15pm UTC](https://boards.straightdope.com/t/how-can-an-organic-molecule-be-stronger-than-metals/474550 "2008-11-26T14:15:08Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\ **Post date:** [November 26, 2008, 2:15pm UTC](https://boards.straightdope.com/t/how-can-an-organic-molecule-be-stronger-than-metals/474550/1 "2008-11-26T14:15:08Z")
I have heard (anecdotally) that pound for pound, silk threds have greater tensile strength than steel. How is this possible? Aren’t the bonds between metal atoms stronger that the bonds between molecules like silk? ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [November 26, 2008, 2:49pm UTC](https://boards.straightdope.com/t/how-can-an-organic-molecule-be-stronger-than-metals/474550/2 "2008-11-26T14:49:05Z")
The trouble is that you are never dealing just with single bonds, and it is the bulk properties that define the eventual measured properties. Spider silk is a nanoscale polymer with a highly complex structure, including protein crystals, elastic joining planes, larger scale shock-absorbing loops and crosslinks. It is constructed at a molecular level by cellular processes, and is also extremely light (low atomic mass). Steel, on the other hand, is drawn from bulk metal stock. However, it is not homogeneous. There are crystalline defects in the bulk structure, and these defects limit the tensile strength. Also, the steel is denser and has a higher molecular mass, so when comparing tensile strength per unit mass, the steel is much thinner than the spider silk, and the defects have a greater effect. If you can make a long defect-free crystal - iron, carbon, etc, you could get a material that is strong enough and light enough to build a space elevator (see A.C. Clark’s _The Fountains of Paradise_, Larry Niven/Steven Barnes _The Descent of Anansi_ for technically detailed sci-fi involving monofilaments) - until then, materials scientists are working on composites that use similar tricks to spider silk to get as close as they can - longer buckytubes, crosslinking, composite binders etc. Si ---
**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\ **Post date:** [November 26, 2008, 2:54pm UTC](https://boards.straightdope.com/t/how-can-an-organic-molecule-be-stronger-than-metals/474550/3 "2008-11-26T14:54:28Z")
If I had to guess, I’d say metal-metal bonds are usually considerably weaker than carbon-carbon bonds. This is because the metal electrons are relatively far away from the nucleous and they are also being held in a higher energy orbital than for carbon. On the otherhand, bonding in a metal is very differrent in that the electrons are held in the conduction band so that every metal atom is bonded to every other atom. I think this is what accounts for metals strength. A strand of carbon-carbon bonds will have strength in one direction, but not another. This can be remedied by cross linking, but then it loses maleability. This is just some educated guessing. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [November 26, 2008, 3:26pm UTC](https://boards.straightdope.com/t/how-can-an-organic-molecule-be-stronger-than-metals/474550/4 "2008-11-26T15:26:17Z")
> [@ralph124c](#): > > I have heard (anecdotally) that pound for pound, silk threds have greater tensile strength than steel. > How is this possible? Aren’t the bonds between metal atoms stronger that the bonds between molecules like silk? In general, the bonds between metal atoms are indeed weaker than bonds between molecules (intermolecular bonds). However, the bonds **within** molecules (intramolecular bonds) are generally vastly stronger than the bonds between metal atoms. Indeed, these are generally the strongest chemical bonds known, stronger than metallic bonds and ionic bonds. So anytime you construct a polymer, which is analogous to one large molecule, you have the potential for a very strong material that can exceed the tensile strength of metals. This is why carbon fibers have such great tensile strength, and is why people are interested in carbon nanotubes. This is also why diamond is so hard, bonded together as it is with intramolecular carbon-carbon bonds. ---
**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\ **Post date:** [November 26, 2008, 3:43pm UTC](https://boards.straightdope.com/t/how-can-an-organic-molecule-be-stronger-than-metals/474550/5 "2008-11-26T15:43:37Z")
> [@robby](#): > > In general, the bonds between metal atoms are indeed **weaker** than bonds between molecules (intermolecular bonds). I think you meant “stronger”. I wonder though how a single metal-metal bond compares to a hydrogen bond. I’d have to dig very deep into my old liquid crystal notes to find the energy of a strong hydrogen bond (pyridine to benzoic acid is one of the strongest I know.) I’ve never seen a calculation of a metal metal bond. I suspect it wouldn’t be very meaningfull, since the metallic strength is really a bulk property. ---
**Author:** ![scm1001](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@scm1001](https://boards.straightdope.com/u/scm1001)\ **Post date:** [November 26, 2008, 9:45pm UTC](https://boards.straightdope.com/t/how-can-an-organic-molecule-be-stronger-than-metals/474550/6 "2008-11-26T21:45:43Z")
the key is pound for pound. A equal sized thickness metal thread will be stronger than a carbon thread. However it will also be 3-6 x heavier depending on the metal. So to compare pound for pound you would have to use a much thinner and so weaker metal thread. There is also a distinction needed to be made between strength and stiffness, which are only indirectly related. The high stiffness of carbon fibres and polymers such as kevlar are due to the alignment and high number or strong carbon-carbon bonds. However the strength is the energy needed to break it, which for a highly brittle sample may be very little (which is one reason we why we dont try and make fibres out of diamond even though it is extremely stiff). Metals have good stiffness, but also being slightly malleable have very good strength usually. ---
**Author:** ![scm1001](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@scm1001](https://boards.straightdope.com/u/scm1001)\ **Post date:** [November 26, 2008, 9:53pm UTC](https://boards.straightdope.com/t/how-can-an-organic-molecule-be-stronger-than-metals/474550/7 "2008-11-26T21:53:48Z")
> [@WarmNPrickly](#): > > I think you meant “stronger”. > > I wonder though how a single metal-metal bond compares to a hydrogen bond. I’d have to dig very deep into my old liquid crystal notes to find the energy of a strong hydrogen bond (pyridine to benzoic acid is one of the strongest I know.) I’ve never seen a calculation of a metal metal bond. I suspect it wouldn’t be very meaningfull, since the metallic strength is really a bulk property. There are many compounds known with " isolated " metal-metal bonds, usually in a cluster of supporting atoms. These are not usually very strong 80-200 kj mol-1 but are fascinating and theoretically interesting. However they dont relate to what goes on in a bulk metal see [Organometallic Chemistry and Catalysis - Didier Astruc - Google Books](http://books.google.co.nz/books?id=aVyfo52-nRsC&pg=PA50&lpg=PA50&ots=rvFbgmQ_yZ&dq=%22strength+of%22+%22metal-metal%22+single+bonds) ---
**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\ **Post date:** [November 27, 2008, 3:34am UTC](https://boards.straightdope.com/t/how-can-an-organic-molecule-be-stronger-than-metals/474550/8 "2008-11-27T03:34:27Z")
I know **scm1001** , I’ve worked with several metal dimer complexes. The metal-metal bond was allways photosensitive. Clearly, bulk metals are not normally photosensitve. Most of the metal-metal bonds I’ve seen though were assisted by bridging species like that Tantalum on page 50 of your cite. When that happens, all bets are off on the type of bond it is. The Chromium species at the bottom of the page though is a perfect analogue of the compounds I worked with. They are all photosensitive and readily break apart homolytically. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [November 28, 2008, 5:01am UTC](https://boards.straightdope.com/t/how-can-an-organic-molecule-be-stronger-than-metals/474550/9 "2008-11-28T05:01:38Z")
> [@robby](#): > > In general, the bonds between metal atoms are indeed weaker than bonds between molecules (intermolecular bonds). > [@WarmNPrickly](#): > > I think you meant “stronger”. :smack: Yes, I did. This should have read, “In general, the bonds between metal atoms are indeed stronger than bonds between molecules (intermolecular bonds)…” ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [November 28, 2008, 5:07am UTC](https://boards.straightdope.com/t/how-can-an-organic-molecule-be-stronger-than-metals/474550/10 "2008-11-28T05:07:43Z")
> [@scm1001](#): > > the key is pound for pound. A equal sized thickness metal thread will be stronger than a carbon thread. I don’t believe this is true. As has been discussed, the covalent bonds between carbon atoms are stronger than the bonds between metal atoms. This is easily demonstrated by comparing the melting point of carbon (3550°C or 6422°F) with that of various metals. For example, iron melts at 1538°C (2800°F). Melting is a bulk property that is dependent on the strength of the bonds between the crystal units making up the solid material in question.