According to the official sources, is the radioactive green crystal that serves as Superman's Achilles' heel actually composed of the noble gas krypton, or is it supposed to be made out of some other substance? If so, what is that substance?
No, kryptonite is not composed of the element krypton.
The composition of Kryptonite varies depending on the source material.
In the Silver to Bronze Age stories as well as in the "Green, Green Glow of Home" episode of Lois & Clark New adventures of Superman Kryptonite is a stable transuranic element (element 126) that finally decays to iron.
However in Superman III (1983) it is said that Kryptonite is a alloy made up of 15.08% plutonium, 18.06% tantalum, 27.71% xenon, 24.02% promethium, 10.62% dialium, 3.94% mercury, and 0.57% of an unknown substance. A Post Crisis story in Action comics #591 (Aug 87) also made Kryptonite an alloy.
No, Kryptonite is not made of the element Krypton. There are a number of versions of Kryptonite over the seven decades Superman has existed, but no matter when it is described, it is almost never composed entirely of the noble gas, Krypton.
Krypton (from Greek: κρυπτός kryptos "the hidden one") is a chemical element with symbol Kr and atomic number 36. It is a member of group 18 (noble gases) elements.
A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquefied air, and is often used with other rare gases in fluorescent lamps. Krypton is inert for most practical purposes.
How did Kryptonite get its name?
- Kryptonite first appeared on a radio broadcast in 1943 and made its first non-green appearance in 1949’s Superman #61, we would not actually see the signature GREEN Kryptonite until 1951 in Action Comics #161.
Kryptonite was needed as a storytelling device to incapacitate the Man of Steel and allow him to have a weakness. Up until then, Superman couldn't be stopped by anything for long.
Given when it was created, most people had a very limited scientific background including the writers, thus its name is likely a very simple fusion of an exotic sounding material (Krypton) and the (-ite) suffix which in chemistry denotes a chemical compound.
- From chemistry: Used to form names of certain chemical compounds, especially salts or esters of acids whose name ends in -ous such as bromite, chlorite, iodite, phosphite, sulfite.
How was Kryptonite described?
Kryptonite's origins vary but most describe its creation at the core of the planet Krypton and were in some continuities responsible for the violent chain reaction which lead to the destruction of that world, killing all of its inhabitants.
During the Silver Age, it was theorized the chain reaction could have been stopped but the Green Lantern assigned by the Oans to deliver the stabilizing agent was delayed by a supernova and did not arrive in time.
Many pieces of Kryptonite arrive on Earth because they traveled near the space-ship Jor-El sent baby Kal-El to Earth in. Other pieces were thought to arrive later.
Some were exposed to various cosmological effects in the Silver Age, Kryptonite came in a variety of colors. Each color had specific capabilities and environmental effects which were caused by exposures to different radiations, natural phenomena or scientific experimentation.
Most depictions indicate it was only the core of the planet which was comprised of Kryptonite, though some of the earlier ones indicated any piece of Krypton that came to Earth was considered Kryptonite.
Kryptonite is a radioactive transuranic element or compound (we are never told which for certain). It's atomic number or compound structure has never been official determined. Okay, there are a few pseudo-scientific attempts at creating a chemical composition but none of them have any basis in reality.*
How does Kryptonite work?
DC's writers have never made an official "scientific" rationalization as to HOW Kryptonite works. Over the decades, we are given a variety of reasons it works, but just vague enough we cannot be exact on the process within his cellular activity.
Kryptonite's inherent radioactivity inhibits the absorption of solar energy which Kryptonians use to catalyze their feats of superhuman ability. Not only does it prevent the further absorption of solar energy, it displaces, painfully, said solar energy with Kryptonite radiation causing the immediate loss of a Kryptonian's superhuman abilities within seconds.
In almost all known continuities, it is Green Kryptonite which has proven to be lethal to Kryptonians who have been exposed to its radiation. Green Kryptonite was lethal to any Kryptonian from the same universe the Kryptonite was derived from.
This fact was discovered by Post-Crisis Superman when he was forced to execute the Phantom Zone villains of another Universe after they had killed everyone on Earth. Post-Crisis Superman was immune to green Kryptonite from their universe and was unaffected by its radiations. (See SE Article: In comics how many people have been killed by Superman?)
This cellular energy displacement of the stored solar energy with Kryptonite radiation can, depending on the purity and exposure length, lead to the death of a Kryptonian by a form of "radiation poisoning". A sign the poisoning is complete is the green cast present in the skin of the now dead Kryptonian.
A nice analogy to describe the effect is Kryptonite functions, in a fashion similar to "carbon monoxide poisoning" where carbon monoxide molecules are being taken up by blood's hemoglobin instead of the normal Oxygen molecules. The CO is displacing the 02 molecules but does not providing any oxygen necessary for life. With sufficient displacement of O2 molecules a person can die.
Uses for Kryptonite (besides killing Kryptonians)
Kryptonite is described as a highly radioactive substance which can be harnessed for energy in an undisclosed matter:
Several of Superman's foes use Kryptonite as part of their arsenal of weapons including Lex Luthor. Luthor was in previous continuities not above using Kryptonite to power many of his exoskeleton weapon systems.
Luthor also wore a ring with a piece of Kryptonite embedded in the ring. Mostly just to piss Superman off. Luthor wore this ring until it gave him cancer. For a smart guy, he could be kinda dumb... Said ring was recovered by Superman and given to Batman, as a part of a contingency plan to "stop" Superman should he ever be mind-controlled or have his free will compromised.
Metallo has a robotic body whose powerplant uses a hunk of Kryptonite as its power sources. Brainiac was also fond of using Kryptonite in many of his schemes including creating a body covered with the material.
More modern depictions of Kryptonite only show small amounts of Green Kryptonite on Earth, but some advanced science has been able to duplicate the signature radiation making it capable of being used as a weapon against Superman.
In previous versions of the DC Universe, the Green Lantern Corps Power Ring was capable of replicating the radiation signature of a piece of Green Kryptonite, potentially making them capable of defeating or at least engaging Superman, if necessary. It has not been revealed if this ability has migrated to the New 52 or Rebirth reboots of the DC Universe.
*Kryptonite in the Real World
- A chemical compound and formula was established for Kryptonite in the movie, Superman Returns in 2006. Luthor described it as: "sodium lithium boron silicate hydroxide with fluorine". It was later discovered in a mine in Serbia and believed to be a fairly unique compound in nature. This material was dubbed Jadarite.
- The new mineral, unlike the fictional material in the movie, does not contain fluorine, does not emit electromagnetic radiation, and is white rather than green (although, in the Superman comics, there is a white colored variety of kryptonite). In all other respects the chemistry matches that of the rock containing kryptonite in the movie. The jadarite fluoresces a pinkish-orange color when exposed to UV light.