09-17-08, 04:04 PM #1
why neutrons are needed
can someone briefly explain why neytrons are needed in the atom ??
09-18-08, 09:17 PM #2
Protons in an atom are all positively charged, and any two positive charges will repel each other electrically.
So, what keeps protons in the nucleus? Answer: there is another force apart from the electric force, called the strong nuclear force. The strong nuclear force between two protons is attractive.
For atoms with many protons, however, the electric repulsive forces between protons are significant - actually too big for the atom's nucleus to remain together, even taking into account the nuclear attractive force between the protons.
This is where neutrons come in. Neutrons have no electrical charge, and so neither attract nor repel each other electrically. But they do feel the strong nuclear force, just like protons. And protons are attracted to neutrons via the strong force, just as they are attracted to other protons.
So, having neutrons in the nucleus creates "extra" attractive force for the nucleus, while not adding to the repulsive forces in the nucleus due to electric repulsion.
For a given number of protons, there is an "ideal" number of neutrons required for a stable nucleus. Too many or too few neutrons lead to a radioactive, unstable atom.
09-18-08, 09:27 PM #3
So if neutrons don't repel each other, why are they just as eager to leave the nucleus?
09-19-08, 10:24 AM #4
09-19-08, 03:29 PM #5
Neutrons have no charge, but they are required by QM as substantial little peices for protons and therefore photons and virtual particles to exist. Without neutrons, the atom would not exist -- for it could not remain stable, and could not hold any gluon energy.
In fact, one small change in the atomic structure, could relate to the catastrophic reduction of all matter as we know it, and reduce to a size smaller than your local football stadium.
09-19-08, 03:30 PM #6
In fact, any smaller, and any reduction, WOULD lead to a reduction in all matter in the wide universe, because there would be no electrostatic balance.
09-20-08, 02:56 AM #7
09-20-08, 03:00 AM #8
09-20-08, 04:47 AM #9
Because experimental evidence tells us that there exists in the nucleus an electrically neutral particle similar in mass to the Proton?
Because we've detected them directly?
Because without them Isotopes are meaningless?
09-20-08, 05:00 AM #10
09-20-08, 12:12 PM #11
Why not a neutral "alpha particle" (four neutrons)? Is the strong force between neutron and proton some how stronger than between two neutrons? I know little about all this, but do not see why nature can not keep adding neutrons to the nucleus? Ie. why does hydrogen with two neutrons (unstable Tritium) exist but not with three? The colection of four barions is very stable but only if two protons and two neutons. These "alpha particle" are so stable that they often survive the violet destruction of large nucli. It dose not make much sense to me.
09-20-08, 07:09 PM #12
09-20-08, 09:54 PM #13
Does any of this explain 'why neutrons are needed'?
Aren't guys simply explaining what happens given that neutrons exist?
You are, probably, answering the questioner in the way he wanted, but it strikes me as a bit teleological.
Like someone saw that atoms could not form without neutrons and thought 'oh, something else is needed'.
They weren't needed. Or....?
09-21-08, 06:23 AM #14
I thought that James R. answered it very well. It's simple:
They knew the charge of 2 protons, However they were staying near each other...even though their charges should repel each other. So something had to be allowing them to stay together, thus the search for the neutron.
09-21-08, 06:34 AM #15
They're needed because they're observed in reality. They're not just some abstract construct invented to make the numbers fit.
As James R pointed out, they're also needed to keep atoms stable - too many protons, you move along the proton drip line, but you can have too many neutrons as well.
09-21-08, 06:40 AM #16
Lemme put it to you another way.
Do you have a better explanation as to why a Hydrogen nucleus has Mass 1, charge 1, mass 2, charge 1, or mass 3 charge 1, where as a Helium nucleus has Charge 2, mass 4, or charge 2, mass 3?
09-21-08, 02:37 PM #17
SUMMARY: As illustrated below, the lighter nuclei may be internally structured to have alpha particle sub groups with a few other simple subassemblies as dynamic structures within the nucleus. I offer some hint that a group of two or four neutrons may be associated as a sub group within the nucleus, at least for the lighter elements. But like the isolated neutron, there is little reason to think that the 2N group is stable outside of the nucleus. I do not see why more neutrons added could not increase stability.
For example, I WOULD THINK Carbon 14 (6P + 8N) should be more stable than stable C13 (6P + 7N), but it is not.
C14 is not even stable! Can anyone offer a reason why this is true?
The following are the largest stable neutron to proton, N/P, ratio for some of the elements (and after the triple dashes, some of my speculations about possible sub nuclear groupings.) If someone who knows the physics of nuclei can comment, I would appreciate it.
D 1/1 = 1.0 --- 0 alphas + D
(D is Deuterium. H is 0/1. --- 0 alphas + P)
He 2/2 = 1.00 --- 1 alpha
Li 4/3 = ~1.33+ --- 1 alpha + H
Be 5/4 = 1.25 --- 2 alphas + N (First example of "nuclear stabilization" of an isolated neutron.)
B 6/5 = 1.2 --- 2 alphas + H
C 7/6 = 1.16+ --- 3 alphas + N
C 8/6 = 1.33+ --- 3 alphas + 2Ns Red as is not stable, despite higher N/P ratio than C13 and also the more common 2Ns sub group.
N 8/7 = 1.14+ --- 3 alphas + H
O 10/8 = 1.25 --- 4 alphas + 2Ns
F 10/9 = 1.11+ --- 4 alphas + D
Ne 12/10 = 1.2 --- 5 alphas + 2Ns
Na 12/11 = 1.09+ --- 5 alphas + D
Mg 14/12 = 1.16+ --- 6 alphas + 2Ns
Al 14/13 = 1.8- --- 6 alphas + D
Si 16/14 = 1.14+ --- 7 alphas + 2Ns
P 16/15 = 1.07- --- 7 alphas + D
S 20/16 = 1.25 --- 8 alphas + 2Ns + 2Ns First time two simple sub-nuclear assembles needed with the alphas? Or more likely (2Ns + 2Ns) = 4Ns is also a stable cluster? (Sort of like an “uncharged alpha” to keep the only one non-alpha sub-nuclear assembly rule longer.) If not as 4Ns, then for the first time, more than one simple structure in nucleus seems to be required, but this isotope is very rare (only 0.016% of all sulfur).
Perhaps S 20/16 (S36) is not really stable – just with a half life too large to observe it decaying? However, as discussed later, it is “doubly magic,” so probably it is stable, or with half life greater than that of the universe? It is very rare and still a small nucleus (8 alphas could be like a little cube). – Too small to easily have only alphas and one 4N sub group? The second most heavy stable isotope of sulfur is (as expected): S 18/16 = 1.125 --- 8 alphas + 2Ns
Cl 20/17 = 1.18- --- 8 alphas + D + 2Ns
This may be the first truly stable isotope with two simple extra structures in the nucleus. It is common (24.6% of all chlorine) but only one other isotope (76.4% of all Cl) is stable: Cl 18/17 = 1.06- --- 8 alphas + D
It is sort of silly, but the 8 alphas could be roughly the corners of a cube with the D on one side and the 2Ns on the other opposite side - almost as if each was the only simple extra structures in the nucleus, to in some sense preserver the "ONLY ONE" simple extra non-alpha structure in the nucleus rule a little longer. This one sub group on two opposite sides idea could also be applied to the doubly magic S20/16. I.e. have each of the 2Ns sub assembles well separtated - sort of like GPS satellites spaced apart as the "fly" around the nuclear core "cube" of 8 alphas.
A 20/18 = 1.11+ --- 9 alphas + 2Ns
K 20/19 = 1.05+ --- 9 alphas + D + P
Or --- 9 alphas + T perhaps Tritium is stabilized here for first time?(To have only one "non-alpha" simple sub nuclear assembly.)
Note strong tendency to have an even number of neutrons in the most heavy stable isotope. (Possibly due to some nuclear spin pairing?) Also if alphas are a substructure within the nucleus, then only Be & C have an unpaired neutron in the most stable isotope in their simple “non-alpha” residue. An isolated neutron is unstable. Clearly neutrons do not “like” to be alone, or even in the company of alpha subgroups, (which can have zero net nuclear spin) in the nucleus.
Also note four different elements had 20 neutrons as their most heavy stable isotope and Ca 20/20 has four but is the lightest stable isotope of calcium. This is an indication that even clusters of 4 neutrons are somewhat favored. S16/16 and Ca 20/20 are stable despite the N/P ratio being only unity, but the N/P ratio never falls to unity again. (Each new proton added to nucleus is repelled by all the others.) These two unity ratio isotopes are “doubly magic” in that both the numbers of Ns and Ps are divisible by four. Cl, A, & K keep their magic 20 neutron groups as protons are added, but Ca can barely do so. (Ca 20/20 is the lightest stable isotope of calcium. I do not know, but possibly it is easy to split – perhaps even a fast electron impacting could do it?) The heaviest most heavy stable, Ca 28/20, surges all the way up to N/P of 1.4, to be the N/P ratio “champ” of all the lighter elements.:
Ca 28/20 = 1.4 --- 10 alphas + 8Ns, Perhaps as four 2Ns or two 4Ns?
Getting quite silly now, but trying to keep my "ONLY ONE" non-alpha sub structure rule: Imagine the 8 alpha cube again. It has four faces which are occupied by two alphas + two 4Ns.
Why should there be such a "ONLY ONE" rule?
Possible answer: The cluster of alphas is very stable and even though knocking into each other in the "nuclear core" they survive to be a "nuclear core" about which the one sub group rapidly "wanders." If there were two of these wandering sub groups, they would occasionally make a "high energy" nuclear physics collision when the do it nucleus decays. -Radio-actrivity of the light unstable atoms is explained by this model also! (especially why often alpha particles come flying out with decay)
Now skipping to the most strongly bound nuclei region:
Cr 30/24 = 1.25 (four stable isotopes. N= 26, 28 & 29 also stable. 28 is “doubly magic” and is almost 84% of all Chrome.)
Mn 30/25 = 1.20 (only this one stable) --- 12 alphas + T + 4Ns, perhaps as two 2Ns
Fe 32/26 = 1.23+ (N= 56 & 57 are also stable. One is the lowest energy / per barion of all nuclei.)
Co 32/27 = 1.185+ (only this one stable)
Ni 36/28 = 2.286- (five stable isotopes 30/26 is lightest. N= 32, 34, 33 & 34 also stable.
Perhaps 14 alphas with 2Ns is 30/28; with two 2Ns is 32/28; with three 2Ns is 34/28 and with four 2Ns or two 4Ns is Nickel 34/28. Ni 33/26 is a problem in this scheme as it has 5 Ns more than the alpha groups. It is 1.25% of all nickel so is not unstable with a long half life (unless some heavier radioactive element is constantly replenishing it as a “daughter” in its decay chain.).
I doubt that the heavy elements* are only sets of alphas with a few other simple units in the nuclei, so I will let someone , who is not just speculating, do a group of the heaviest elements, and just note Uranium has 92 protons and 99.2 % of U238 has 146 neutrons for N/P ratio of 1.587- and U235, the fission fuel has 143 neutrons for N/P = 1.554+ which means that neutrons are supplying less binding to keep the Coulomb forces from destroying the nucleus. Neither is stable but clearly in some structural sense U238 is more so as reactor do not “burn” it.
*Possibly some larger groups, which are common in fission products are subassemblies in the U235 nucleus?
Later by edit: Some text is now orange to aid others find my discussion of "nuclear stabilization" of neutrons and perhaps Tritium, etc.
Last edited by Billy T; 09-22-08 at 08:10 AM.
09-21-08, 06:55 PM #18
There's a theory about an island of stability beyond the unstable transuranium and transactinide series.
They recently found element 122 occurs naturally, in thorium. That was this year, btw.
Also, check out "halo neutrons" in isotopes; neutron pairs can form Borromean rings - a tertiary structure which is more stable somehow.
09-21-08, 08:18 PM #19
I'm a chemist, rather than a nuclear physicist, so i'm not going to claim to have more then a rudimentary understanding.
It's my understanding that the current leading theory effectively predicts interlaced shells of protons and neutrons, much like with the electronic structure.
Essentialy, you have the equivalent of a first proton shell, and a first neutron shell, but these first shells overlap.
Particularly stable nuclei occur at certain numbers of Neutrons, Protons, or both, for much the same reason that chemically stable elements occur - because half full and completely full nuclear shells are more stable then shells with just one nucleon in them.
It's my understanding that it's this magic number theory that explains things like the island of stability, and other features such as the stability of Technitium, and the stability of some isotopes over others.
09-21-08, 10:07 PM #20
To add another aspect I don't believe has been mentioned, a free neutron is unstable, beta decaying to hydrogen with a half-life of ~14min iirc. So in order for a stabile isotope to exist their must be a process such that neutrons in the nucleus are stabilized against decay. And since beta decay occurs in some isotopes this stabilization isn't perfect. It is reasonable to suppose that certain arrangements are disallowed on this account.
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