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Let's say that I have a special sword that has macroscopic maximum thickness, and has a minimum thickness of 1 atom. It's also incredibly wear-resistant, which would make it usable. Let's say that I swing it about through Earth's atmosphere, which is 78% nitrogen, 21% oxygen, and contains trace elements like carbon dioxide and water. The sword is 2 m long, and light enough to be lifted. With a blade that cuts chemical bonds, what's the most dangerous chemicals I can produce with that sword alone?

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    $\begingroup$ Let's put a bound on "Dangerous"- most corrosive? Chemically Reactive? Explosive? Lowest LD50? Rack up the most insurance claims? $\endgroup$
    – PipperChip
    Mar 18 at 20:02
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    $\begingroup$ just because the blade can cut chemical bonds does not mean the wielder can see or hit chemical bonds well enough or frequently enough to get anything of any quantity. $\endgroup$
    – John
    Mar 18 at 20:08
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    $\begingroup$ I'm reasonably confident that the sword wouldn't cut molecular bonds. The molecules will elastically rebound from a blade even an atom thick. After all, the molecules are themselves an atom thick in profile, and colliding with atom-thick particles at great speeds all the time. It's not impossible, but stretching it. Any free atoms that were released would be highly reactive, so you might generate a few stray free radicals, but no more than sunlight. $\endgroup$
    – DWKraus
    Mar 18 at 21:13
  • $\begingroup$ What do you mean by "cutting" chemical bonds? Do you think that chemical bonds are made of pieces of string, which can be mechanically cut with a knife? $\endgroup$
    – AlexP
    Mar 18 at 22:06
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    $\begingroup$ @ChristopherJamesHuff That's the point I'm trying to make. An atom-wide sword isn't going to be able to cut molecules like the OP's question seems to indicate. Unless you start adding in magic, the chemistry & physics just don't work. I didn't have exact speeds, but knew it was crazy-fast compared to a sword. $\endgroup$
    – DWKraus
    Mar 19 at 4:06
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Awesome, wimpy power.

There aren't many bonds in normal air to cut: O-O, N-N, maybe H-OH if there's some humidity. So you get isolated neutral O and N atoms, and maybe H and OH on occasion.

The isolated atoms have more energy than bound ones, so the most striking thing about your sword is that it's a perpetual motion machine. Either that or it has to pay the energy cost, in which case it has more resistance to swinging through the air than any other sword in the entire history of brutes with pointy objects.

But how much of an effect is this? Well, air at a comfortable temperature is 1 mol per 24 liters... oh, forget that, we don't need it. You cut bonds in maybe an angstrom slice of air ($10^{-10}m$). (Molecules can be wider than that, but you'll hit many edge on) For each mighty swing that averages a meter of distance times a meter of sword, you create $1m \times 1m \times 10^{-10}m = 10^{-7}L = 10^{-4}cm^3 = 0.1mm^3$ of monooxygen/nitrogen/etc. A tenth of a cubic millimeter ... I'm quaking in my shoes. The energy produced (or required) is $0.1uL / (1 mol /22.4 L) \times (946 kJ/ mol N2 bond) = 4.22 mJ$. (Oops, we did need it) That's about $1/250$ of a watt-second (like a kilowatt-hour but smaller), or 1 millionth of a food "Calorie", so your swordsman doesn't need to stock up on energy drinks to swing that thing around.

If it can cut nuclei in half, then we might have something. Random subsets of protons and neutrons from O and N nuclei probably make some pretty spectacular radiation, and it wouldn't take very much... oh wait, I've been reminded that the sword would have to cut the nucleus, which no matter how sharp it is, is still absurdly unlikely.

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  • $\begingroup$ Nuclear diameter is on the order of 10^-15m for small elements. Using your formula, adjusting by a factor of 10^-5 and plugging in the nuclear binding energy (5.3 * 10^10 kJ/mol), you're still looking at...62J? Equation source $\endgroup$ Mar 18 at 22:08
  • $\begingroup$ Oh. Then there's that. I should have remembered the area the blade had to strike would get smaller. :) Can we get a redesign where you can swing the broad side of the blade and any nucleus in the area gets taken up and rearranged for zero energy? Thanks. :) $\endgroup$ Mar 18 at 23:43
  • $\begingroup$ "macroscopic maximum thickness, and has a minimum thickness of 1 atom" <- The OP seems to already allow for a sword of normal width, just give it billions of parallel mono-molecular edges and you should be able to sweep a 2-3mm wide path even without cutting on the flat of the blade. $\endgroup$
    – Nosajimiki
    Mar 19 at 2:35
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    $\begingroup$ @Nosajimiki Sure, but if the cutting part is the width of an atom, that's still ~4.5 logs wider than the nuclei. That's like trying to split a hair lengthwise by swinging the Washington Monument at it $\endgroup$ Mar 19 at 16:51
  • $\begingroup$ Re: spectacular radiation. Yes, but not as much as you want. Air is not very dense, and neither are atoms (though the definition starts to break down at that scale). The cross-sections of N and O are very small, so you probably wouldn't have enough interactions to produce any radiation noticeably above background. Also, I suspect a lot would be alpha, which would be shielded by the simple expedient of having skin $\endgroup$
    – Sol
    Mar 19 at 18:24
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Just chemically breaking down the elements in the air is not that big of a deal, Mike's answer does a good job of outlining why... but let's pretend the sides of you sword also contain complex mono-molecular geometry, something similar in shape and function to RNA, then it could take those loose elements and reform them into new molecules. Since you have the technology to make a monomolecular blade, I will assume you can do this as well. Now your sword is its very own a chemistry lab activated by the kinetic energy of your swing.

Because thermodynamics is always ruining every world builder's fun you can not use your swing to form a significant amount of high explosives or anything fun like that because the energy you store in any created molecular bonds can not exceed that of your sword swing.

This leaves you with toxins. The two deadliest toxins in the world by volume are Botulinum toxin and Maitotoxin, but both of these contain elements that you would not abundantly find in the air, but the 3rd deadliest toxin is Batrachotoxin (C31 H42 N2 O6). This toxin could theoretically be synthesized from Carbon Dioxide, Nitrogen, and Water Vapor which are all relatively abundant in the air. A lethal dose of Batrachotoxin is just 136 μg.

Since no cut has truly perfect edge alignment, even a monomolecular blade can sweep a few millimeters width of air as it moves. If you you swing your blade around carelessly, it will accumulate this poison on the flat of the blade kind of like condensation. That said, the really deadly part will be when it actually cuts into a person. Flesh has a much higher molecular density than air, so just passing through a couple of mm of skin will liberate relatively large amounts of the elements you need to poison and kill your opponent.

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  • $\begingroup$ A sword that turns its victims to poison. Nice! $\endgroup$ Mar 18 at 21:49
  • $\begingroup$ @TysonDennis - with the understanding that this blade is straight-up magic. $\endgroup$
    – jdunlop
    Mar 26 at 23:27
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    $\begingroup$ @jdunlop The same could be said for any monomolecular blade. Frankly, a blade capable of triggering RNA like chemical reactions is IMO more plausible than one that can kinetically cut through molecular bonds. We can already make nano-materials from proteins; so, being able to make proteins from nanomaterials should not be that far fetched. $\endgroup$
    – Nosajimiki
    Mar 29 at 13:56
  • $\begingroup$ Nosajimiki - The lack of a power source would make chemical assembly difficult. That said, yes, that's my point. There's no magic tag in @TysonDennis's post, but pretty much all of the answers are actually describing magic swords, because monomolecular edges wouldn't last. $\endgroup$
    – jdunlop
    Mar 29 at 17:30
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Let's set aside the plausibility of a blade really cutting atomic bonds, and let's take for good that it happens. Any singlet atoms that you would produce is not exactly friendly, considering that it will desperately try to recombine because chemistry. Considering both abundance and reactivity, I would say that the nastiest thing that you will produce is atomic oxygen

An excited state of molecular oxygen generated photochemically or chemically. Singlet oxygen reacts with a variety of biological molecules such as NUCLEIC ACIDS; PROTEINS; and LIPIDS; causing oxidative damages.

Molecular oxygen is already reactive, atomic oxygen is even more reactive than that.

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Not much most of the time

Air is really close to local energy minimum. So Expected would be many momentary bond cleaving events just to have them re-form as they were shortly thereafter.

Sword will likely rust into dullness

So to create say monatomic oxygen momentarily might cause some interesting reactions but I would expect re-combine into standard diatomic oxygen most often. Second often would be reacting with the sword to rapidly rust away the edge until the chemical bond cleaving stops.

It is a catalyst

The sword is essentially just a very good non specific catalyst. So what does a car's catalytic converter produce? Generally it reduces that amount of dangerous chemicals, same with this sword.

The most dangerous I would expect out of this setup would be things like: Carbon monoxide, ozone, nitrogen oxide. And those would be in very small quantities.

Not the right conditions

If you think it would spontaneously create H-N=N-N=N-N=N-H or some such? No, for same reason as shattered tea-cups don't spontaneously jump into your hand complete with tea.

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  • $\begingroup$ Had to look up azidotriaziridine (H2N6). I think it's dangerous. Can someone prove that? $\endgroup$ Mar 18 at 21:51
  • $\begingroup$ I just made up a long chain of N because I could type it in and the longer the chain the more unstable and dangerous. It is almost guaranteed to be explosive. It is in the difficult and dangerous to create category so would be hard to find literature. $\endgroup$ Mar 18 at 22:49
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This is a sort of follow-up to Nosajimiki's answer.

Poison is not a useful thing in melee. Batrachotoxin is indeed very deadly, but it isn't instantly lethal or debilitating. Someone cut with and poisoned by a Batrachotoxin blade could continue to fight for a minute or more. Plenty of time to run you through with his own blade before he died. Poison doesn't help you win a sword fight, it just lets you turn a defeat into a Pyrrhic victory. The best thing about your fancy monomolecular sword will be its amazing cutting properties, it should go through bone like butter.

If you absolutely have to use this poison idea, put it on an arrowhead or something, so you can run away and wait for the poison to take effect.

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  • $\begingroup$ This assumes it is an unarmored dueling sword. As a battlefield weapon, they make a lot more since. Cavalry often favored swords for doing pass-by attacks. Here you are already not sticking around for a counter attack; so, instant kills don't matter so much. Armored infantry benefit even more. Armored infantry battles often dragged on for many hours with very few people suffering more than superficial wounds until one side either got too tired or broke formation. If both sides have have swords that can nick you through armor, but not fatally penetrate it, poison becomes very useful. $\endgroup$
    – Nosajimiki
    Mar 22 at 15:39
  • $\begingroup$ And even if you turn a defeat into a Pyrrhic victory on an individual level, doing this many times over the course of a battle can turn the whole course of the conflict in your favor. Because if the first guy in your formation dies having only scratched his opponent, that opponent is now going to be much easier for your second guy to take out. $\endgroup$
    – Nosajimiki
    Mar 22 at 15:45
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From Wikipedia's article on the atmosphere, there are five main compounds that you can cut in the air are:

  • Nitrogen (78.08%) -- Will be cut into atomic nitrogen, which likely just wants to recombine back into N2

  • Oxygen (20.95%) -- Will cut into atomic oxygen, which may just react with almost anything. Likely it will react back into O2 gas

  • Carbon Dioxide -- Cutting one of the chemical bonds will get you carbon monoxide and a free oxygen atom. The lone oxygen atom will react with something and be begging to do so. Not sure if it would be able to just react back into CO2 or not. Due to its linear bond structure, it is likely impossible to cut both oxygen atoms from the Carbon.

  • Methane -- Possessing 4 C-H bonds in a sort of caltrop-like shape, cutting them will basically strip a number of hydrogen out of the gas, leaving an incomplete hydrocarbon behind. Likely, it will be cut into a CH3 group and an hydrogen atom.

  • Water Vapour -- One of the H-O bonds get cut here, not unlike with CO2. This nets a hydrogen atom and an hydroxide unit. Unlike with CO2, the molecule is not a straight line, so there might be a slim chance to slice both hydrogen atoms off the water molecule.

The other major components of the atmosphere are the noble gasses, and they will not form molecules that can be cut under normal conditions, so that is roughly 0.934% of the atmosphere that has no bonds to cut (Argon mostly).

From that list above, the most dangerous thing is probably the nitrogen or the oxygen, but likely it will just react with itself again. That typed, there are plenty other trace materials that are in the atmosphere in minute quantities.

  • Dust, Pollen, and Spores probably won't cut into anything super deadly. However, is is likely the most interesting thing that you could cut the bonds of. The larger molecules of organic matter offer up more ways to be cut.

  • Chlorine and Fluorine could be present in trace amounts and be cut, but like with atomic oxygen, the halogens will likely react before they pose any real danger to anything or anybody. Fluorine in particular just wants an electron and will mug almost anything to get it.

  • Sulphur compounds can be present, with hydrogen sulphide and sulphur dioxide being the two common ones that get thrown into the air. Due to the angled bond in the molecule, the same premise as with water works for them.

Now the answer is that the most dangerous thing you an probably make is atomic oxygen on a regular basis. You wouldn't really be able to make enough carbon monoxide to be dangerous, and anything truly nasty is only there in trace amounts

Now if your sword could store what it cut, or gave you control over what it cut meaning that you the wielder has control over how the cut molecules recombine, that would be a different matter entirely.

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As others have shown, it's not going to do anything relevant. It's not organized so you can't make a toxin with it, all you can do is chop off radicals.

L.Dutch came up with the most aggressive radical that will be produced in any quantity but there's another that you'll get on occasion that makes a lone O look downright friendly: A lone F. There's no fluorine gas in the atmosphere but there are fluorine-bearing compounds on particles at the parts-per-billion level.

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  • $\begingroup$ "It's not organized" <- If you mean the blade geometry, the OP made no such stipulation. In fact, since you can't exactly "cut" molecular bonds anyway, I think it's safe to assume that it would have to be very organized in order for it to chemically break air molecules as it passes. So much so that the same technology could inherently also be used to assemble molecules too. $\endgroup$
    – Nosajimiki
    Mar 19 at 2:51
  • $\begingroup$ @Nosajimiki No--I was saying the chopping isn't organized. You can chop off pieces but you can't control how they reassemble. $\endgroup$ Mar 19 at 2:53
  • $\begingroup$ Given the concentration of fluorine compounds, and the reactivity of free fluorine, you're not going to have a loose F for long enough for it to be a threat - and you're not going to make enough of it to present a risk to an opponent. $\endgroup$
    – jdunlop
    Mar 26 at 23:26
  • $\begingroup$ @jdunlop Quite true. Since there's no way to make anything that's actually a threat with the blade I was trying to get the technically most dangerous. $\endgroup$ Mar 27 at 23:37
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It's probably not the worst thing that could be created, but your blade is probably going to produce at least some cyanide gas as a byproduct. As your magic sword cuts CO2 and N2 in the atmosphere, you're going to have some C4+ and N3- ions that are going to be really, really eager to combine with each other and reach an electrically neutral state, and will end up forming CN- ions, and will then probably bond with an H+ ion for good measure to produce gaseous hydrogen cyanide.

Now, I don't know what percent of carbon and nitrogen ions cut by the sword will end up joining with one another, or how much cyanide gas will end up being produced (let alone if it will be enough to be lethal), but's its something that will end up being produced in some amount just assuming random atomic collision.

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