90
votes
Accepted
Why do fusion and fission both release energy?
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 ...
- 1,551
86
votes
Accepted
Why is hydrogen the most abundant element in the Universe?
The short answers are that:
(i) Protons (hydrogen nuclei) are produced abundantly in the early universe but only a small fraction of these are able to engage in nuclear reactions leading to heavier ...
- 136k
77
votes
Accepted
Can there be an atomic nucleus where there are more protons than neutrons?
What you are looking for is isotopes with neutron–proton ratio N/Z less than 1. You can find these isotopes, for example, in this list from Wikipedia. As you can see, you are looking for members of ...
70
votes
Why do fusion and fission both release energy?
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 ...
- 97.8k
65
votes
Accepted
At what temperature are the most elements of the periodic table liquid?
We take the natural elements (atomic numbers $1$ to $92$) to have well-defined melting and boiling points. Additionally, all figures are quoted at the standard $1\text{ atm}$ pressure. Here's some ...
- 8,570
56
votes
If iron can’t undergo fusion, does that mean a black hole is mostly iron?
If we are talking about stellar-sized black holes, then the object that collapses to form a black hole will have a high concentration of iron (and other iron-peak elements like manganese, nickel and ...
- 136k
44
votes
Can there be an atomic nucleus where there are more protons than neutrons?
According to Wikipedia:
Other than protium (ordinary hydrogen), helium-3 is the only stable isotope of any element with more protons than neutrons.
- 49.4k
44
votes
At what temperature are the most elements of the periodic table liquid?
I thought another visualization of the data set might be interesting. Here's the liquid range for each element up to and including uranium, sorted vertically from lowest to highest boiling point. (...
- 1,857
43
votes
If iron can’t undergo fusion, does that mean a black hole is mostly iron?
Iron can undergo fusion. However, iron is the point where fusions starts to cost more energy than it yields, so in a typical star it doesn't fuse.
In a supernova, and the abundance of energy ...
- 560
43
votes
Accepted
Why is helium-4 the only nuclide with a negative nucleon binding energy?
Helium-4 won't accept a neutron to form helium-5, and it won't accept a proton to form lithium-5: both of those isotopes have ground states which decay by alpha-nucleon fragmentation. But you are ...
rob♦
- 94.2k
42
votes
Were alchemists right?
They were wrong in the same way the people who made human-sized wings to fly were wrong. The goal of flight/metal transmutation is not impossible, but the methodology is naive and hopeless.
- 428
38
votes
Accepted
Why do "relativistic effects" come into play, when dealing with superheavy atoms?
When quantum mechanics was initially being developed, it was done so without taking into account Einstein's special theory of relativity. This meant that the chemical properties of elements were ...
- 30.2k
37
votes
Accepted
Are there individual protons and neutrons in a nucleus?
It's a fair question. Let's start with the related question:
Are there individual electrons in an atom?
Here the answer is generally "no." The answer is more emphatically "no" ...
rob♦
- 94.2k
34
votes
Accepted
Were alchemists right?
There are ways that gold can be produced by radioactivity:
Chrysopoeia, the artificial production of gold, is the symbolic goal of alchemy. Such transmutation is possible in particle accelerators or ...
- 235k
32
votes
Why do fusion and fission both release energy?
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 ...
- 979
30
votes
How it was found that 12 g of carbon-12 has Avogadro's number of atoms?
This Chemistry.SE answer captures most of the argument, but I'll paraphrase here. (Oh, who am I kidding. The history of units and conventions is never short. "Paraphrase" should be taken ...
- 51.7k
28
votes
Why, precisely, is argon used in neutrino experiments?
There are many different types of neutrino detectors, using various techniques to turn a neutrino interaction into an electrical signal. Detectors that use a large quantity of water or oil, like Super-...
- 35.8k
23
votes
Are elements above 137 possible?
Empirically, nuclei composed from magic numbers of nucleons ( either proton number or neutron number, separately) which follows Otto Haxel's relation
$$a(n) = \frac{n(n^2 + 5)}{3} \tag 1$$
Starting ...
- 14.9k
22
votes
Are elements above 137 possible?
For a point nucleus with $Z \gt 1/\alpha$ the Dirac equation has no ground state solution for a single electron, that is a +(Z-1) ion. Itzykson and Zuber describe this catastrophe on page 75. For a ...
- 26.6k
21
votes
Accepted
When will hydrogen no longer be the most abundant nucleus?
We can only give an answer on the basis of what we currently know about cosmological parameters. If indeed these have been correctly estimated, and that the cosmological constant is constant, then the ...
- 136k
21
votes
Why do fusion and fission both release energy?
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 ...
- 15.3k
21
votes
Were alchemists right?
There are two aspects to this. First, what kind of physical process can transform one chemical element into another. Second, the contribution of alchemy to the development of science.
On the first ...
- 61.8k
21
votes
How it was found that 12 g of carbon-12 has Avogadro's number of atoms?
It was defined this way:
In 1971, the International Bureau of Weights and Measures (BIPM) decided to regard the amount of substance as an independent dimension of measurement, with the mole as its ...
- 65k
19
votes
Cobalt atomic mass less than 100 % isotope mass?
The mass of an atom is actually a little bit smaller than the sum of the
masses of its constituents (protons, neutrons, electrons).
The missing difference is due to the negative binding energy of the ...
- 41k
17
votes
Accepted
Why do different elements have different number of isotopes?
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 ...
- 16.7k
16
votes
Why is hydrogen the most abundant element in the Universe?
Between about t=$10^{-12}$ and t=$10^{-6}$ seconds, the universe was filled with a quark-gluon plasma. Temperatures were too high for mesons and baryons (like protons and neutrons) to remain bound, ...
- 578
16
votes
Why do elements, as they have more protons, need a higher amount of neutrons to stabilise them?
The nuclear force only acts between nearest neighbors to hold them together- but the electrostatic force driving them apart has infinite range, so any one proton in the nucleus feels the repulsion of ...
- 97.8k
15
votes
Why do "relativistic effects" come into play, when dealing with superheavy atoms?
So this is not a coincidence, but is in fact, rather fundamental. With the Heisenberg Uncertainty Principle:
$$ \sigma_p\sigma_x\ge \frac{\hbar}2$$
if a particle is confined to a space less than:
$$ \...
- 39.5k
13
votes
Why is the number of isotopes of an element bounded?
I think this is a good question -- after all, if there's no extra Coulomb repulsion penalty for adding more neutrons, unlike for protons, why can't nuclei have lots of neutrons?
One model for the ...
- 2,456
13
votes
Accepted
What might be the half-life of observationally stable nuclei with energetically favorable decay modes?
There is the empirically found Geiger-Nuttall law
relating the energy of the $\alpha$ decay to its half-life:
$$\log T_{1/2} = \frac{A(Z)}{\sqrt{E}}+B(Z)$$
where $T_{1/2}$ is the half-life, $E$ the ...
- 41k
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