75

In general, both fusion and fission may either require or release energy. Purely classical model Nucleons are bound together with the strong (and some weak) nuclear force. The nuclear binding is very short range; this means that we can think of nucleons as "sticking" together due to this force. Additionally the protons repel due to their electric charge. ...


63

Fission releases energy, because a heavy nucleus (like Uranium-235) is like a cocked mouse trap: it took energy to squeeze all those protons and neutrons hard enough together to make them barely stick (by the nuclear force) against the natural tendency for all those protons to fly violently apart because of their electrostatic repulsion. When struck by an ...


38

In a nutshell, atoms decay because they're unstable and radioactive. Ununoctium (or Oganesson) has an atomic number of 118. That means that there are 118 protons in the nucleus of one atom of Oganesson, and that isn't including the number of neutrons in the nucleus. We'll look at the most stable isotope of Oganesson, $\mathrm{{}^{294}Og}$. The 294 means ...


38

There are a few different ways that unstable nuclei are produced: Nuclear fusion is quite a common way to produce unstable nuclei in nature. At high enough energies, stable nuclei can fuse together to create unstable ones. For example, one step of one of the usual hydrogen-burning sequences in stars combines a helium-3 nucleus and a helium-4 nucleus (both ...


31

As far as the theory goes, you are absolutely correct, the (negative) binding energy between atoms in a molecule contributes to the total mass of that molecule, so a stable molecule is less massive than the sum of the masses of its constituent atoms. However (as you yourself calculated), the mass difference is absolutely tiny, and as far as I know, it has ...


29

From Wikipedia: Iron-56 (56Fe) is the most efficiently bound nucleus meaning that it has the least average mass per nucleon. However, nickel-62 is the most tightly bound nucleus in terms of energy of binding per nucleon. (Nickel-62's higher energy of binding does not translate to a larger mean mass loss than Fe-56, because Ni-62 has a slightly higher ...


28

Your assumption about the lowest energy state when everything is tightly stuck together is incorrect. It only goes this way until you get iron nuclei - and this is why iron is the heaviest element created by fusion. Creating nuclei heavier than iron consumes energy rather than releasing it. And this is why these elements are only created in supernova ...


23

This is actually a more complex question than you might think, because the distinction between mass and energy kind of disappears once you start talking about small particles. So what is mass exactly? There are two common definitions: The quantity that determines an object's resistance to a change in motion, the $m$ in $\sum F = ma$ The quantity that ...


23

The "folk wisdom" that iron-56 has the highest binding energy per nucleon is in fact incorrect; both iron-58 and nickel-62 have a higher binding energy per nucleon, with nickel-62 being the highest. I can't do much better than citing an article on the subject: M. P. Fewell, "The atomic nuclide with the highest mean binding energy". Am. J. Phys. 63, 653–...


22

The existence of nuclei is dependent on a number of quantum mechanical boundary conditions. They appear as solutions to a problem where there is a balance of: a) the attractive spill over color force that binds the quarks into a proton or a neutron, b) the repulsive electromagnetic force between protons, c) the Pauli exclusion principle, d) the instability ...


21

Great question! My answer would be that in order to get a bound state, we need to have a potential that is deeper than the kinetic energy the two particles have. We have a better chance of getting a potential of the right depth to bind two nucleons if: (1) They don't repel charge-wise. Compared to nucleon interaction the Coulomb force isn't that strong, ...


21

A lot of different forms, but mostly kinetic energy. A good table is given at Hyperphysics. The energy released from fission of uranium-235 is about 215 MeV. This is divided into: Kinetic energy of fragments (heat): ~168 MeV Assorted gamma rays: ~15-24 MeV Beta particles (electrons/positrons) and their kinetic energy: ~8 MeV Assorted neutrons and their ...


20

Indeed, the range of possible decay times is much, much wider than even the range you've given, as I found out myself recently. It's hard to imagine a physical quantity that varies more! In a big-picture sense, one way to think about this is the following: radioactive decay can be thought of crudely as a form of quantum tunnelling, where the nucleons tunnel ...


20

I wanted to add another answer to show an important plot - binding energy per nucleon versus the atomic number (number of nucleons [protons + neutrons]). The binding energy is the amount of energy required to break apart a nucleus. If, after a change, the amount of binding energy decreases, we must have supplied energy to break apart a nucleus. If, on the ...


19

There is an electrostatic repulsion between the protons in the nucleus. However, there is also an attraction due to another kind of force besides electromagnetism, namely the so-called "strong nuclear interaction". The strong nuclear interaction ultimately boils down to the forces between the "colorful" quarks inside the protons - and neutrons. It is ...


17

Iron is a "special" element because of its nuclear binding energy. The very basic idea is that when you fuse two light elements together, you get a heavier element plus energy. You can do this up to iron. Similarly, if you have a heavy element that undergoes fission and splits into two lighter elements, you also release energy. Down to iron. You can see ...


17

The Sun obviously produces far more energy per second than is required to fuse an iron nucleus with some other nucleus. The problem is concentrating all that energy on the iron nucleus. It's not enough to know that it takes the energy from $n$ hydrogen fusions to fuse one iron nucleus, it's getting the energetic products from those $n$ hydrogen fusion events ...


17

The main difference is gonna be the stability of the various isotopes. Most elements technically have a very large number of isotopes (carbon isotopes range from carbon 8 to carbon 22), but most of these have a very short half-life due to the poor stability of a number of neutrons too large (or too small). The list of isotopes will usually be either somewhat ...


17

There's no fundamental principle that makes unstable states unable to exist. It's just that by being unstable, they won't exist for a long time. For example, take a cone. You could sit the cone on a table with its base at the bottom, and that would be stable ("stable" here means that if there is a small perturbation, the object settles back to its original ...


15

I want to offer a different perspective from the already existing answers, which all seem to somehow refer to the Standard Model or other specific physical theories to say that mass is not an integral multiple of some fundamental mass unit, hence not discretized. The reason why mass is not like that - and can indeed conceivably have continuous values in a ...


14

If they didn't release energy, they wouldn't happen. The alternative, nuclear reactions that require energy, clearly need said amount of energy, which has to come from somewhere, e.g. kinetic energy involved in the collision of two nuclei (even ones that release energy usually have a "barrier" and some amount of initial kinetic energy is needed to overcome ...


13

If I'm reading rightly, I think your main question is: Why does only a small percentage of rest mass turn into energy [even for fusion]? It's because the universe is very strict about a certain small set of conservation rules, and certain combinations of these rules make ordinary matter extremely stable. Exactly why these rules are so strictly observed ...


13

The most obvious interpretation of the phrase blow up the Earth is to dismantle it into tiny particles headed off to infinity. If you're prepared to accept this definition then the calculation is easy because it's (approximately) the gravitational binding energy for matter with the mass of the Earth falling into a sphere the size of the Earth. I say ...


13

Heavier nuclei can also undergo fusion, but that's not very useful for energy production. One of the reasons is, as you've mentioned, the binding energy per nucleon. Let's have a look at the binding energy curve (image taken from Wikipedia): Iron-56 has the highest binding energy per nucleon, which means it is the most stable nucleus. Roughly speaking, ...


13

Why do atoms decay at all? For the same reason that rocks roll downhill. There is a general tendency for things that are at a high energy level to "fall" to a lower energy level. In terms of atomic nuclei, the lowest energy per nucleon is iron (Fe-56). Energy can be released by fission of elements heavier than iron and fusion of elements lighter than iron. ...


11

When people say that the decay rate depends critically on the $Q$ value, they're talking about alpha decays compared to other alpha decays. When you compare alpha decay to emission of other small clusters, the dependence on the atomic number $Z_c$ of the emitted cluster is much more prominent. The reason is as follows. In the Gamow model of beta decay, we ...


10

To understand binding energy and mass defects in nuclei, it helps to understand where the mass of the proton comes from. The news about the recent Higgs discovery emphasizes that the Higgs mechanism gives mass to elementary particles. This is true for electrons and for quarks which are elementary particles (as far as we now know), but it is not true for ...


10

The nucleon-nucleon interaction has a short range, roughly 1 fm. Therefore if there were to be a bound dineutron, the neutrons would have to be confined within a space roughly this big. The Heisenberg uncertainty principle then dictates a minimum uncertainty in their momentum. This amount of momentum is at the edge of what theoretical calculations suggest ...


10

The important argument for this discussion is the Bethe Weizsäcker formula, which describes the binding energy of nuclei. I will try to give a cursory overview of the most important aspects. Not only heavy elements show fission and fusion. All elements up to iron-56 (one of the nuclei with the highest binding energy per nucleon) can create energy in fusion (...


10

Fusion: In a small nucleus there is a relatively large fraction of nucleons at the surface, which lowers the total binding energy. The fusion of 2 very small nuclei to one medium-sized nucleus releases energy, mainly because in the resulting bigger nucleus there are fewer nucleons at the surface than before. This is analogous to the surface tension effect ...


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