# What single element could destroy the world?

What single chemical element (get out your periodic tables) could most efficiently destroy all life on the planet.

Restrictions:

• Natural elements only, and no anti-matter...nice try.
• Looking for the element that a mad scientist would need the least of to wipe out all vertebrate life.
• How much would they need? (This is the efficiency part. I want to use the least amount of element x as possible)
• This needs to be achievable by near future means, no magi-tech, no hand-wavium.
• Where would the element come from? (can it be harvested or manufactured?)
• By what process would the world be destroyed?

Please do not include more than ONE scenario in your answer. Looking for depth on this one.

• Ironically, it takes far less cyanide by mass (remembering that energy=mass) to kill all vertebrates, than any of the answers so far listed here. – RBarryYoung Jun 3 '15 at 16:25
• As RBarryYoung suggests, going the (somewhat more boring than the suggestions below...) route of something simply poisonous is going to be a much more efficient approach, than if you want explosions etc.. Even so, the answer will probably be something radioactive, perhaps Polonium. (Though, if we answer How much would they need by mass, the super-light but really toxic Beryllium might be a candidate.) – leftaroundabout Jun 3 '15 at 23:01
• Note that both of the most upvoted answers, while technically true, are totally useless for the purpose of the question: to create the basis of a realistic story. – vsz Jun 4 '15 at 13:13
• While not an element, using quantum entanglement to (instead of teleporting an object in whole), teleport random bits of an object to random entangled particles at random places in the cosmos, would be an effective means of obliteration. – Kraang Prime Jun 10 '15 at 5:17
• @HDE226868 If you use a single element to start a chain reaction/process that is acceptable. – James Jun 12 '15 at 13:34

## Hydrogen

But by chemical means, not relativistic ones. Relativity is LAME.

Hydrogen is highly flammable in oxygen. Luckily, we're living at the bottom of a giant pile of oxygen, so all we need to do is mix in the hydrogen thoroughly and light 'em up.

But how much hydrogen do we need? First, we need to know the formula for combustion of hydrogen: $2H_2+O_2 \to 2H_2O$. Now we need to know how much oxygen there is in the atmosphere: $$5.15\times10^{18}kg * 20.25\%\ O_2 * \frac{1000mol}{32.00kg} = 3.26\times10^{19}mol\ O_2$$Convert that to the mass of hydrogen: $$3.26\times10^{19}mol\ O_2\ * \frac{2 mol\ H_2}{1 mol\ O_2} * \frac{2.02kg\ H_2}{1000mol\ H_2} = 1.32\times10^{17}kg\ H_2$$

So all you need to do is create or find 130 quadrillion metric tons of hydrogen (I recommend collecting it from interstellar space, or synthesizing it from seawater through electrolysis, which would only take 18.6 yottajoules or 1.54 quintillion gallons of gasoline), release it into the atmosphere, and wait for a tiny spark or fire anywhere on the Earth. If something the incredible explosion doesn't kill it, the lack of a single molecule of oxygen anywhere on the planet surely will. And if it survives that, there's a flood of $$1.32\times10^{17}kg\ H_2 * \frac{1000mol\ H_2}{2.02kg\ H_2} * \frac{2mol\ H_2O}{2mol\ H_2} * \frac{18.02kg\ H_2O}{1000mol\ H_2O} * \frac{1m^3\ H_2O}{1000kg\ H_2O} = 1.17\times10^{15}m^3\ H_2O$$ coming to drown them.

• Side note: 15.4m gallons of gas costs around $46 million, so this is probably the cheapest option so far. – evankh Jun 3 '15 at 9:21 • nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html – ch7kor Jun 3 '15 at 10:09 • Total mass of atmosphere: 5.1 x 10^18 kg If you really want to burn all oxygen you need to have 10^10 times more hydrogen. There is plenty of it in space. However If you use electrolysis to get it from the ocean, you will have enough hydrogen but you will create also corresponding oxygen. – ch7kor Jun 3 '15 at 10:17 • See's the light 'em up link... "Please be the Hindenburg, please be the Hindenburg" clicks link... Fall Out Boy?! makes sock puppet accounts to mass downvote – corsiKa Jun 3 '15 at 15:46 • But you need to mix it up really well. Otherwise it won't explode, just burn. And the water won't "come down", it will be vaporous. It will probably rather vaporize some more water from the Oceans than condense. Live wouldn't primarily be destroyed by fire/explosion, it would be destroyed by lack of oxygen and high temperatures in the sense of a messed up climate. – Nobody Jun 5 '15 at 12:16 Hydrogen. You can destroy the world with less hydrogen than with any other element, because it's available in smaller quantities than any other element. Of course, it's not easy to destroy the Earth with a single atom of hydrogen. It is, however, possible. We just need to get our hydrogen atom moving fast enough. Specifically, we need to get it to a speed where it has about as much energy as there is binding energy in the Earth. At this speed, it will hit the edge of the atmosphere and start colliding with other particles, creating lots of other absurdly high energy particles as a result of collisions. This wave of extremely high energy particles will fuse with other atoms in the air along a fusion shockwave travelling at close to the speed of light. The shockwave will eventually stop fusing everything in its path as it slows down somewhat, but it will have more than enough energy in it to utterly obliterate the Earth. How fast do we need to get the hydrogen atom moving? We can calculate the energy of our hydrogen atom by using the equation for kinetic energy of a relativistic particle:$K = (\gamma-1) m c^2$, where$\gamma = 1/\sqrt{1-v^2/c^2}$. With the mass of a single proton, the velocity required for that proton to have enough energy to destroy the Earth is a mere$(1 - 10^{-84})c$. Of course, that much energy will not only kill all life on Earth, but destroy the planet itself. A somewhat slower hydrogen atom could be used if the goal was merely to trigger a shockwave strong enough to destroy all life on the surface or knock it into an orbit where the sun would incinerate all life. • what-if.xkcd.com/20 – apaul Jun 2 '15 at 22:52 • You would have to make sure it does not simply go through the earth though. – PyRulez Jun 2 '15 at 23:42 • While clever, I don't think this is a very good answer. The amount of energy in that proton, is equal to that of annihilating 590 billion tonnes of antimatter. If you could harness 100% of the sun's energy striking Earth and could convert it perfectly into proton kinetic energy, it would take 19 million years to get the proton up to speed - seems a little outside the capabilities of the average Mad Scientist. Now granted, a boom big enough to destroy the world is going to need about that much energy no two ways about it. But merely wiping out all life would be many orders of magnitude cheaper. – Blake Walsh Jun 3 '15 at 9:02 • @BlakeWalsh I think you're underestimating just how mad this scientist is. – ckersch Jun 3 '15 at 12:41 • I just want to say I'm disappointed that even with Lorentz contractions the supermassive proton looks to still be 10^20x too large to collapse into a black hole. – David Moles Jun 3 '15 at 22:39 Moving in the other direction from ckersch... A neutron star is an atom the size of a mountain. As they explain in the [reference], neutron stars are "like giant atom cores" that are "unbelievably dense and violent." They are exactly like atom cores with one notable exception: atom cores are held together by strong interaction between subatomic particles, while neutron stars are held together by gravity. and here is a video of that nucleus destroying Earth. 1. Neutron stars are natural objects, humans are incapable of producing them. 2. Neutron star would wipe out the Earth. 3. The minimum stable neutron star mass is ~$ 1.44 M_{\odot} $4. Neutron stars are the remains of supernovae as are most exotic elements 5. The Earth would be destroyed by gravitational disruption. Description of the element The atomic mass of this star would be calculate using the following: Mass of Sun ~$M_{\odot} =1.99 \times 10^{30} kg $Mass of neutron star$ = 1.44 \times M_{\odot} = 2.86 \times 10^{30} kg $Mass of neutron ~$ 1.67 \times 10^{-27} kg $Atomic mass of star ~$ \frac {2.86 \times 10^{30} kg}{1.67 \times 10^{-27} kg} = 1.7 \times 10^{57} $About 10% of neutron star mass is actually protons, so Atomic number of star$ = 0.1 \times 1.7 \cdot 10^{57} = 1.7 \times 10^{56} $This concept might win in the "fewest atoms" required to do the job category but it is going to lose if you use the "amount of matter" as the determinant. • This is really a bit of a stretch. And besides, wouldn't you agree that a neutron star contains a lot more mass than would be needed for pretty much any element? – HDE 226868 Jun 2 '15 at 22:55 • It depends upon perspective. A neutron star really is a mass of neutrons with a few protons and electrons thrown in. Usually they have a non-degenerate crust. But a neutron star has the same density as an atomic nucleus. From many perspectives it is an atomic nucleus - which makes it an element. Which element is anyone's guess. – Jim2B Jun 2 '15 at 23:33 • @HDE226868 But you only need 1.660539×10^-24 mols of it. – PyRulez Jun 2 '15 at 23:44 • Okay, I relent. +1 for thinking way outside the box. – HDE 226868 Jun 3 '15 at 12:40 • Related: What is the most dense object in the universe? on the Astronomy Stack Exchange. – a CVn May 17 '16 at 21:28 Aluminium - Build a solar shade or solar shades which block most of the light from reaching earth. Temperatures will plunge far below what is survivable, the entire surface will freeze and the Earth will become a snowball, even after the shades move out of position the highly reflective ice will keep the Earth frozen. All mammals will definitely perish, and most surface vertebrates. The aquatic ones will be harder to kill especially those which depend on deep sea vents, but under any scenario it's going to be really hard to get deep sea fish at least within a 1000 year timespan. In any case, wiping out all surface life is pretty much guaranteed. How much would you need? I think about 10-100 million tonnes. It's doable with near future technology, and a total surface kill is pretty much guaranteed. I can think of some excellent "molecular" solutions to wipe out all surface life with possibly lower mass requirements and possibly even within the resources of a single madman, but those are molecules of 2-3 elements. In terms of a single element, a super thin sheet of aluminum in the right place does the trick. Here are some references to solar shade solutions: Solar Radiation Management on Wikipedia L1 Solar Shades (these are transparent) LEO Solar Shade Rings (pdf) High Altitude solar shade balloons (pdf) - contains lots of useful stuff about the properties of metal for reflectors and so on. Note all those articles are about trying to help the world. The mad scientist would just need to do it about 10x better than the proposals. He could initially pretend to be performing one of these schemes to help the world in order to get the required industrial resources and political support. • See now this is mad scientist worthy. – James Jun 4 '15 at 16:30 • I feel it would be slightly more mad scientist to use the aluminum to build a giant parabolic mirror deathray :D. Which would then proceed to do the opposite (keep deathraying the earth so it heats up enough to turn into Venus). No clue which of both options is more efficient though (partly 'cause life's a bitch to kill). – HSquirrel Jun 5 '15 at 7:20 • @HSquirrel triggering a runaway greenhouse effect is the obvious alternative to snowball earth, and I considered it too. One problem is it would take a damn long time to melt Greenland and Antarctica making the polar latitudes a safe haven - the thermal mass of the ice will locally counteract the warming for a long time. In a sense, Antarctica gives us a head start on creating snowball Earth. – Blake Walsh Jun 5 '15 at 7:26 • @Roughcoat Well the possible locations are Earth-Sun L1 point, in orbit around Earth, or high altitude balloons. The L1 point will offer the most comprehensive shading, as other shading solutions will only shade at best 50% of the time. The cheapest would be high altitude balloons, but they might get shot down... Surface wont work well because both nature and people will rip the shades up and destroy them. – Blake Walsh Jun 6 '15 at 13:27 • If the aluminum sheet was 1 micron thick, you'd need 3800000 Meters^3. That would weight 10,256,200,000 kilograms or 10,256,200 tonnes. It would be easier if you used a cloud of smaller disks. If each disk had a half meter diameter, you'd only need 16 trillion of them. You could do it by firing a capsule containing a million shades into space every 5 minutes for 10 years using 20 separate launch sites. – AndyD273 Jun 8 '15 at 15:39 # Astatine Astatine is a member of the Halogens family, and it is a really nasty radioactive element. Just getting a visible amount would cause so much heating due to radioactivity that it would vaporize. Now it likes to collect in the thyroid, so if enough collects there peoples' heads explode right off of their necks, due to the Astatine vaporizing. Astatine is also a halogen, which means it is highly reactive, so that means that it will combine with other elements and possibly form into some kind of noxious gas. • Alternately, pick any of the transuranics, magically create a couple of kilos of it, and stand well back. By the time the resulting multi-explosion finishes (R Munroe, of XKCD fame, describes it in his book What If? as "They don't just decay, they decay into things that also decay - a veritable salad of everything in the Periodic Table turning into everything else as fast as possible") there aren't likely to be many survivors... – Shadur Jun 3 '15 at 8:57 • Hmm... no. There is simply no way that enough Astatine could accumulate in the thyroid to cause any kind explosion. If you are exposed to significant amounts of Astatine, you'll be dead from acute radiation sickness long before your thyroid even comes into play. – DevSolar Jun 3 '15 at 12:29 • "Astatine is also a halogen, which means it is highly reactive" Astatine is less reactive than iodine, which is not terribly dangerous from a reactivity point of view. – psmears Jun 11 '15 at 15:05 All it takes is a little push in the right direction. It's not so much the element you have, it's what you apply it to... When you're looking at some of the really destructive things nature can wreak all on it's own, a really small explosive charge disturbing a natural balance can be all it takes. And you can get explosions (ok ok, conflagrations) from a wide variety of elements which I don't even have to list. Let's say you have a natural nuclear fission reactor. Drop a packet of the right stuff at the right place, disturb the balance, and have it go "boom!". Let's say you have a supervolcano in the making, with a nice big dome. Think Yellowstone. If the magma is depressurized even a bit, it will gas out, setting off a chain reaction that makes the whole thing go "boom!". Those are the two "global killers" I could come up with right away. For more localized destruction, think fault lines (setting off earthquakes), mountain flanks (setting off megatsunamis), thinks like that. • As a sidenote, all it took for the Chernobyl reactor to go firecracker was a couple of pounds of Xe135... – DevSolar Jun 3 '15 at 13:14 • The real culprit of the Chernobyl incident was a poorly constructed, poorly maintained, and undermanned reactor. What it really took was even more ubiquitous than Xe135. It only took an ounce of greed and stupidity. – Drunken Code Monkey Jul 26 '17 at 1:59 # Sulfur But use it to freeze the earth, not for burning. You only need on the order of $$10^{11}$$ kg. And all the science is available today. I'm cross-posting this answer from How much Sulfur Dioxide is needed to freeze life on earth?, which was inspired by this question. ## What is the extinction mechanism? I’m going to guess that changing the average temperature from 16 degrees Celcius to -20 degrees will cause enough havok to create mass extinctions. The average temperature during the last ice age was about 10 degrees. If the temperature near the equator ranges from 10-30 above the average, -20 globally would get the temperature there dipping below freezing regularly. This is inspired by Intellectual Venture's hose-to-the-sky cooling scheme for combating global warming, described in Levitt and Dubner’s SuperFreakonomics, where all my citations are coming from. Intellectual Venture’s plan to completely reverse global warming requires 5 base stations, placed strategically across the globe, each with 3 hoses (p. 196) spraying liquified sulfur dioxide into the stratosphere, 7 miles up (p. 189). Each hose sprays at 34 gal/min (p. 192), or 190 kg/min, so 2800 kg/min for all 15 hoses. Since they said this will “effectively reverse global warming” (p. 196), let’s assume that 2800 kg/min will decrease the average temperature of the earth by 2 degrees Celcius. ## How much sulfur is needed? I’m guessing that sulfur dioxide injection would be affected by the law of diminishing returns, but I’m going to assume a linear relationship here. With that assumption, decreasing the average global temperature 36 degrees would require pumping about 100,000 kg of sulfur dioxide per minute. To cause mass extinction, let’s say we have to run this for two years, to make sure that artic and antarctic animals don’t get a chance to stock up on more food in the “summer”. That will take $$10^{11}$$ kg of sulfur dioxide, or $$5*10^{10}$$ kg of sulfur. Do we have that much sulfur handy? It looks like it. The Athabasca Oil Sands in Alberta, Canada have pyramids of sulfur as a waste product of oil extraction. Leavitt and Dubner's book describes pyramids “a hundred meters high by a thousand meters wide” (p. 195), or 30 million cubic meters = $$6*10^{10}$$ kg S in each pyramid. How many hoses do our five pumping stations need? Each station needs to output 20,000 kg/min, which would require about 100 hoses at each site, or maybe bigger hoses. If we assume just two giant pumping stations, at the Athabasca site and at a similar site somewhere in the southern hemisphere, the sulfur dioxide would cover the earth in about 10 days (p. 194). Together, they can freeze the earth for four years, assuming the earth cools off quickly once it can’t receive as much energy from the sun. The sulfur dioxide would settle out of the atmosphere “within a few years” (p. 197). ## How much would this cost? Intellectual Ventures estimates \$150 million in opening costs and \$100 million per year thereafter for their modest "Save the Earth" proposal (p. 197). Our "Destroy the Earth" operation is about 18 times bigger, so I will guess about 18 times the cost: in the neighborhood of 3 billion dollars for opening costs, and 2 billion dollars to run it per year thereafter: it costs just 7 billion dollars to freeze the earth for two years. Well within the reach of your typical multi-billionaire mad scientist. ## But sulfur dioxide is not an element... For those of you who are going to argue "but sulfur dioxide is not an element", the OP specifically states that he is "Looking for the element that a mad scientist would need the least of", and that element is sulfur. You could just as easily argue that burning hydrogen is not a valid answer because that would produce hydrogen dioxide (water), and water is not an element (at least in modern chemistry). If an element is causing a lot of death, it is probably because it is that element is undergoing a chemical reaction. If my arithmetic is correct, this scenario is too close to plausibility for my comfort. • Welcome to the site browly. Nice answer. – James Jan 20 '16 at 20:22 At the time I posted this, the question did not yet specify "near-future", and indeed made no reference to technical feasibility. With the updated question, this answer is no longer really relevant, but might still be interesting from a technical standpoint, so I leave it open. You said only one scenario per answer, so I guess I have to write up a second answer. You ruled out antimatter, so the most destructive is nuclear reaction. And here it gets surprisingly easy: ANY element, as long as it's a really unstable isotope of it. Well, OK, it's not mine. It's a variation of what HSchmale already posted, and something I picked up elsewhere (XKCD? PeriodicTable.com? Cannot recall, really.). So here we go. In a nuclear fission bomb, only a fraction of the fissile nuclei actually participate in the chain reaction. Now imagine you take a unstable isotope of whatever, really. Let's say you have some magic, or sufficiently advanced technology, to create, let's say a pound of the stuff, right now. Thanks to the really short half-life, all of its nuclei undergo decay at, for all practical purposes, the same time. You just got the equivalent of a multi-pound nuclear warhead. That's fun, let's do it again! This time with a ton of the stuff... And you don't have to get fancy to get a chain reaction, because this is a natural reaction, full stop. As soon as the stuff is there it will decay, and the only upward limit to destructive force is how much of that isotope you can create before the (instant) destruction of everything around it brings an end to your creating it. • Only a few radioactive isotopes can sustain a nuclear chain reaction.$U_{233}U_{235}$&$Pu_{239}$are a few (I heard once that one of the$ Po $isotopes could too). Of those,$ P_{239}$is the most energetic and$U_{233}$is the least. The difference isn't that big though (2-3%). – Jim2B Jun 3 '15 at 13:23 • It looks like there's something like 18 fissile isotopes. But due to stability, ease of machinability, and other issues; the two Uranium and the one Plutonium isotope are preferred. – Jim2B Jun 3 '15 at 13:33 • @Jim2B: You've missed the point. For my answer to work, you don't need a chain reaction, because the nucleus will decay naturally. Take Beryllium-8, with a half-life of 6.7×10^-17 seconds. Twenty pounds of Beryllium-9 on your table will give basically the same result as twenty pounds of Uranium-235 undergoing chain reaction. The only difference is that it's much easier to get the Uranium and then trigger the chain reaction than it is to get the Beryllium-9 in macroscopic quantities (thank god), but feasability wasn't part of the question. ;-) – DevSolar Jun 3 '15 at 13:44 • @Jim2B: The whole point of the U / Pu chain reactions is to get lots of nuclei to fall apart at roughly the same time, releasing lots of energy. With an unstable isotope of really short half-life, you get just that, simply because of the inherent instability. However, because of the instability, you'd have to "magic" all those nuclei into being at the same time for it to work. – DevSolar Jun 3 '15 at 13:51 • Right, but what I was pointing out was that some of these other isotope decay at much faster rates. Making it difficult to assemble sufficient quantities of the isotope to get a nice "bang". There're problems with beginning introducing neutrons too soon (you get a fizzle) or too late (materials already moving apart, so not enough of them fission). I'm not saying you can't do it, just saying it's much harder with other isotopes. – Jim2B Jun 3 '15 at 14:01 There are a lot of extreme-tech answers which do not meet the near-future-tech requirement of the question. Amongst the other answers I see: The hydrogen approach that's got a 10 orders of magnitude error. (Edit: It has been fixed. Now I'm using a lot less material than he is.) The aluminum answer is a lot more viable but I recall seeing numbers on a sun shield and I have the definite feeling the numbers were a lot higher. The Astatine answer doesn't work--the half life is too short--you can't collect it in your thyroid. Besides, it couldn't explode--long before it reached that level of heat it would cook the tissues and no more would come in at that point. Anyway, so long as you'll accept not getting the sea mammals I've got an answer that will do it with much less material. Specifically, about 300 million kg of cobalt-60. Scattered at the rate of 2 mg/m^2 this will produce a dose of .8Sv/day. That should be enough to get a kill of all higher land life forms. Obtaining that quantity of cobalt-60 is going to be a considerable problem as it only comes about as a fission product or by neutron activation of cobalt-59. We don't really know the economics of it's mass production as it has very limited use (radiation experiments and non-destructive sterilization--although if we could get over the fear factor this might be a substantially bigger market, the production of shelf-stable versions of things that normally must be refrigerated) so I can't say what it would take to actually produce it. • How much$Co_{60}$does the Earth have? – Jim2B Jun 4 '15 at 18:00 • Loren can you elaborate on your answer a bit more. Like take into account surface area of land on the globe and get more specific on the amount of cobalt-60. It would also help if you could explain where that would be gotten/synthesized. – James Jun 4 '15 at 18:09 • You really should use comments (or edits) to comment on or improve other answers. The only portion of this answer that actually answers the question seems to be about the last two lines of text; the rest is commentary on other answers. – a CVn Jun 5 '15 at 9:17 • @James If you take, say, fifty H-bombs in the hundred megaton range and jacket them with cobalt thorium G, when they are exploded they will produce a doomsday shroud. A lethal cloud of radioactivity which will encircle the earth for ninety three years!...It is not a thing a sane man would do. The doomsday machine is designed to to trigger itself automatically. – Aron Jun 6 '15 at 8:17 • Wikipedia has a good article on the Cobalt bomb. – DevSolar Jun 9 '15 at 6:25 Carbon (with a smidge of oxygen and maybe some hydrogen). All you need is a really virulent virus, made from those elements. The quantity needed is miniscule. A single breath might contain more than a million times the quantity needed. • Single element Henry. – James Jun 9 '15 at 20:14 How about Element zero? Phil calls it "the doomsday explosive" in this short video. See time index 5:26 : if the stasis field on this baby were to fail, it would release the energy equivalent of fifteen million Tzar Bombs in the first second. And in the second second, it would release the energy equivalent of fifteen million Tzar bombs. And so it would continue for the second and the third and the forth second... and it would only be after ten minutes that the energy release from this one cubic centimetre (he's British —Ed.) of neutronium would subside to releasing the energy equivalent of only seven million Tzar bombs every second. Needless to say, I want one. That certainly qualifies as "least amount" in volume anyway. It's naturally occuring, as explained in the video, in substantially larger quantities than the marble being discussed. Technically, it's an element: each neutron individually is an atom of Element Zero, and the sample is not bound together (it's too small for gravity), so it's just rather concentrated, like gas under pressure is still the same stuff just easier to store and transport. Oxygen a surprisingly innocent but effective choice. All you got to do is increase the ratio percentage in the atmosphere enough, that starting a fire causes a global fireball. If that doesn't kill everyone, the environmental consequences should finish them off. • Nope. Everyone dies way before that: Oxygen in high concentrations is toxic. – Nobody Jun 6 '15 at 18:55 • I think you get fires spreading a lot more before you reach concentrations that cause oxygen toxicity. – Jarred Allen Apr 2 '17 at 14:26 Hydrogen Not by using any of the other methods already described but by using it in fusion. The only way to be certain you have destroyed all life on Earth is to destroy the Earth. As a lesser objective still likely to kill all life on Earth, boil the oceans. Based upon this answer to another World Builder question, the energy required to destroy the Earth is$2.2 \cdot 10^{32} J $. The energy required to boil the Earth's oceans is ~$ 5 \cdot 10^{26} J $. By using Hydrogen and it's isotopes in a fusion reactions, the Earth's oceans possess a total of$ 5.3 \cdot 10^{34} J $worth of energy. What mass of hydrogen do we need to destroy the Earth? ~$ 6.25 \cdot 10^{17} kg H_2 $(about 0.5% of the hydrogen in Earth's ocean). What mass of hydrogen do we need to boil the Earth's ocean? ~$ 6.25 \cdot 10^{11} kg H_2 $(about 5 billionths of the hydrogen in Earth's oceans). The smaller of these numbers ($ 6.25 \cdot 10^{11} kg $~ 260 Giga kg ~ 260 Megatons of Hydrogen). Strange Matter If the Strange Matter hypothesis is true, then a tiny bit of strange matter would eventually destroy the Earth. If the strange matter hypothesis is correct and its surface tension is larger than the aforementioned critical value, then a larger strangelet would be more stable than a smaller one. One speculation that has resulted from the idea is that a strangelet coming into contact with a lump of ordinary matter could convert the ordinary matter to strange matter.[13][14] This "ice-nine"-like disaster scenario is as follows: one strangelet hits a nucleus, catalyzing its immediate conversion to strange matter. This liberates energy, producing a larger, more stable strangelet, which in turn hits another nucleus, catalyzing its conversion to strange matter. In the end, all the nuclei of all the atoms of Earth are converted, and Earth is reduced to a hot, large lump of strange matter. You could make a "strange element" (called a strangelet) out of strange matter composed of up + down + strange quarks. Restrictions: Natural elements only, and no anti-matter...nice try. Looking for the element that a mad scientist would need the least of to wipe out all vertebrate life. How much would they need? (This is the efficiency part. I want to use the least amount of element x as possible) This needs to be achievable by near future means, no magi-tech, no hand-wavium. Where would the element come from? (can it be harvested or manufactured?) By what process would the world be destroyed? 1. Strange matter would be natural in the sense that it exists somewhere in the Universe without human intervention but it is not native to the Earth (that we know of) and we wouldn't normally think of it as an element. 2. If the Strange Matter Hypothesis is true, than a tiny bit (one to a few particles) of it would slowly convert all normal matter it comes in contact with, into strange matter. This will destroy the Earth. 3. You would make strange matter in a particle accelerator. I do not know the size required to make it (e.g. CERN might not be powerful enough). 4. Two mechanisms would work in concert to destroy the Earth. 4a. First, the conversion from normal matter to strange matter would liberate vast quantities of energy. Early in the process, these effects wouldn't be noticed but towards the end, the Earth would begin to glow like a star. 4b. Second, the conversion would vastly increase the density of the matter. It would turn the Earth into a stable substellar neutron object. If the Strange Matter Hypothesis is true, then this concept would win the contest on the amount of mass required to destroy the Earth but it would not disrupt the Earth. Rather, it'd collapse the Earth to be a very small object <$\frac{1}{1000000}\$ the volume of a neutron star. It would become the densest object possible without forming a black hole.

• How do you figure the size of strange matter? I thought it was only slightly denser than neutronium, in the color-flavor locking phase. – JDługosz Jun 4 '15 at 12:19
• "Strange Matter" is a flavor (or color, I get them mixed up) of quark. You are correct, a strange matter object would be slightly denser than regular neutronium. But the hypothesis I mentioned states posits that it is a more stable form of matter. When it comes in contact with normal nuclear matter, it converts normal matter into strange matter spontaneously. – Jim2B Jun 4 '15 at 12:32
• strange is a flavor. You said it was 1/100000 the size of a neutron star. I question that. For that matter, why doesn't the strange matter at the core of a "quark star" eat the surrounding neutronium and crust? – JDługosz Jun 4 '15 at 18:11
• Not 1/1000000 the density but 1/1000000 the size. This is based upon the Earth possessing about 1/1000000 the mass of the sun. Really it's something more like 1/7000000 the size & mass of a neutron star. – Jim2B Jun 4 '15 at 18:47
• and the Strange Matter Hypothesis to which I'm referring isn't the most widely accepted interpretation of how things work. The broader and more widely accepted interpretation is that it requires the high pressures found at the center of a quark star to remain stable. – Jim2B Jun 4 '15 at 18:49

Edit: When I first wrote this I didn't grok the "single element" statement fully.
I'm not really changing my answer, just changing the execution.
With the right planning and equipment, you could wipe out the majority of life on earth with only 120-210 kg of Uranium/Plutonium.

Take the uranium and shape it into cores for bombs of around 30kg, which IIRC can deliver a yield in the 50-100 kiloton range with the right design.

This is the most efficient way I can think of to trigger the release.

You could probably do the same job with less plutonium. It's surprisingly hard to find statistics for core sizes and weapon yields.

No one has mentioned Methane yet.
Methane is heavier than oxygen and nitrogen, which means that it stays close to the surface instead of rising into the atmosphere, and there is enough of it stored in hydrates under the ocean to cover the surface of the earth and suffocate the majority of people.

Methane is also a very powerful greenhouse gas, which means the earth would warm significantly very quickly, and so the oceans would rise, and the gas would rise...

The best part is that because the methane is already stored in the hydrates you wouldn't need to make any, you would just need a submarine to go down and release it with some digging or bombs, which is pretty Mad Science.
The scientist would only have to release a fraction of the stored methane, which would cause a lot of warming, which would heat the oceans, releasing more methane.

The idea that this could happen naturally is called the Clathrate Gun Hypothesis.

And then there would be the opportunity to ignite the methane...

The check list:

Natural elements only, and no anti-matter...nice try.

Just normal weapons grade uranium or plutonium...

Looking for the element that a mad scientist would need the least of to wipe out all vertebrate life.

This would wipe out all surface life, though the earth itself would remain. The majority of people/animals would suffocate very quickly, and the methane explosions would probably get the rest.
You might get a handful of survivors at high altitudes for a while, but not enough to sustain a population.

How much would they need? (This is the efficiency part. I want to use the least amount of element x as possible)

Not that much. Enough for 4-6 medium size bombs. Rough calculations say 120-210 kg of uranium (30 kg per core), 60-100 kg of plutonium (15-20 kg per core).

This needs to be achievable by near future means, no magi-tech, no hand-wavium.

We could do this now with simple technology. It would just take a bit of money to map out the richest deposits and then build the nuclear bombs. Probably dig a few oil well style holes down into deposits, and then insert the bombs into the wells. Like fracking the ocean floor.
This could happen by itself at any time, if there was an earthquake in the wrong place.

Where would the element come from? (can it be harvested or manufactured?)

Africa probably.
I have heard of a nice guy in Libya that can get you some if the price is right, and I've heard of a guy in Chad selling yellow cake over the internet...
Russia has potential too.

By what process would the world be destroyed?

Suffocation, immolation, starvation.

• Only problem is that methane is not a single element. I like the idea though. – James Jun 8 '15 at 16:11
• @James That is a good point... It is a single molecule, but not an element in it's own right. I may have to think on this more. – AndyD273 Jun 8 '15 at 17:06
• If you could generate a process where you introduce one of the elements to a process that creates methane non-stop that would fit I think. – James Jun 8 '15 at 17:50
• @James Plutonium is probably the single element key. I don't know if I should rewrite the answer or not. Doing some rough calculations, lets say 120kg of plutonium separated into cores for 4 bombs, placed strategically around the globe in the largest hydrate deposits using submarines, and set off at the same time. I had a comment about making methane using Nickle, but I don't really think you could make enough fast enough. A few relatively small explosions in the right places could do the job though. – AndyD273 Jun 9 '15 at 17:09

Only the tiniest amount of Silicon, built into nano-processors, embedded into the hulls of the tiniest nanotech robots, and programmed to create more robots out of all available nearby resources. ;)

Short, but sweet.

• As clever as the grey goo scenario is, it already happened. It's colloquially referred to as the "green goo" scenario, also called "life." (That is: organic matter already does better at reproducing using all available resources than tiny silicon or iron based robots). – Draco18s no longer trusts SE Jan 20 '16 at 20:11