soma
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atomic weight of iodine
Iodine has an atomic number of 53. I thought that would mean it has 53 protons, 53 neutrons, and 53 electrons. The atomic weight of iodine is 127. So
somehow it got 21 extra neutrons?
[Edited on 29-4-2016 by soma]
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diddi
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neutrons do not have charge, so it is not essential that they "balance" the presence of another type of particle. it is very common, particularly in
larger atoms, for there to be quite a number more neutrons than protons in the nucleus. in fact there can be a variable number of neutrons for a given
element, which give rise to isotopes.
Beginning construction of periodic table display
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soma
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Wondering what causes that particular number of neutrons. I would assume there has been some theory as to why it happens that way.
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Marvin
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Atomic weight of iodine is 127 according to google.
Atomic number doesn't tell you about the neutrons, only the protons. The more protons in a nucleus the higher the number of extra neutrons needs to
be to get all that charge to stick together.
Hydrogen has 1 proton, H-1 (protium) is stable without a neutron, H-2 has one and H-3 has 2 but is radioactive.
Uranium has 92 protons but isn't stable enough to exist as a solid until it has about 140 neutrons.
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Fulmen
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https://en.wikipedia.org/wiki/Neutron%E2%80%93proton_ratio
Simply put it has to do with the electrical repulsive effect between the protons and the attractive strong nuclear force.
We're not banging rocks together here. We know how to put a man back together.
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aga
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I got all confoozeled with this, and someone (j_sum1 ?) pointed out that the elements have a variable number of isotopes which can been seen on
ptable.org if you click Isotopes. (no, i never clicked that before either)
Iodine has 37 known isotopes, so there can be 37 different number of neutrons in it with 126.90447 g mol<sup>-1</sup>being the
average atomic weight (or something like that).
The # of protons & electrons remains at 53, just the number of neutrons may be greatly different.
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DraconicAcid
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Don't ever assume that the number of neutrons is equal to that of the protons. It works for some of the lighter elements, but not for anything past
calcium.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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soma
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I'm wondering how easily the number of neutrons can change and how tightly the nucleus holds on to them.
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DraconicAcid
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It can"t easily change. If it does change, that's a nuclear reaction.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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diddi
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in nature each element is represented by a number of "versions". as has been pointed out already, they proton count determines the element type. ie
every atom that has 53 protons is iodine. it will look and behave chemically as iodine. the neutrons act as a filler in the nucleus. after all if
the nucleus contained all positively charged particles they would repel each other and no be stable at all. the neutrons sit between the positively
charged protons to provide enough distance so the protons do not eject each other, but are no so distant that the nuclear bonds are too weak and the
atom falls apart.
so there is a critical limit of too few and too many neutrons for each element type. the range for small elements is only small, but for larger ones
there might be quite a few in the range that allow for the atom nucleus to be stable.
@aga
the average you talk about is worked out by giving the isotopes which are more common, more importance. so for iodine, the common one is 127, but the
others contribute slightly to the "average" and so the atomic mass is a little different from 127. But for chlorine as an example, there are 2 very
common isotopes and the atomic mass quoted on a periodic table is split almost right between the 2 at 35.5.
Beginning construction of periodic table display
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annaandherdad
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diddi---good explanation. I would add this: The strong force between the protons and neutrons is short range, about 10^{-13} cm, so protons and
neutrons feel the strong force mostly when they are in contact with one another, and they don't feel much of a strong force with their more distant
neighbors in the nucleus. The strong force dies off exponentially with distance. The electric force, however, is felt at a distance, since it only
dies off as the inverse square of the distance. The electric force is weaker than the strong force, but as you add protons to a nucleus, the
electric force accumulates, while the short-ranged strong force does not. So when you add protons you have to add even more neutrons to glue them
together, and the neutron to proton ratio gradually increases in stable nuclei as the number of protons increases. However, if you add too many
neutrons, then it becomes unstable again. This only works up to a certain number of protons, after which it is impossible to make a stable nucleus no
matter how you adjust the number of neutrons. That is, the nucleus becomes radioactive. Adding even more protons (and neutrons) you get
progressively more and more unstable nuclei.
sima---it's fairly easy to change the number of neutrons in a nucleus, you can just send in some neutrons and they may "stick". Since the neutrons
are neutral, they don't feel any repulsion from the nucleus, and you can just let them drift in a even a very low energy and they will get there.
They don't actually "stick"; what they may do is get absorbed, producing a new nucleus with a larger number of neutrons, possibly in an excited state.
The new nucleus may very well be radioactive, even if the old one was not. The neutrons may also just simply bounce off the nucleus (elastic
scattering).
Any other SF Bay chemists?
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Fulmen
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Good explanation, annaand. When it comes to isotopes and chemical/physical properties the general rule is that they are virtually identical. This is
because chemical properties are defined by the electron number and the net charge. And since the number of electrons follow the number of protons not
much happens if you add or subtract a neutron. There is a slight increase in mass that will change physical properties just as slightly, but in most
cases you can't even notice this.
The only exemption is for the lightest elements, especially hydrogen. If you add a neutron to H-1 (producing deuterium or H-2) you effectively double
the mass. While the electron configuration is the same, the increase in mass is so significant that it affects both chemical and physical properties.
We're not banging rocks together here. We know how to put a man back together.
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unionised
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Quote: Originally posted by diddi  | in nature each element is represented by a number of "versions".
.
@aga
the average you talk about is worked out by giving the isotopes which are more common, more importance. so for iodine, the common one is 127, but the
others contribute slightly to the "average" and so the atomic mass is a little different from 127.
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In the particular case of iodine, the number of versions is one.
Only one isotope is stable so essentially all the iodine you see is 127 iodine.
The difference between the exact mass and 127 is due to something else.
https://en.wikipedia.org/wiki/Nuclear_binding_energy#Mass_de...
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soma
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Thanks for the replies.
Now I'm wondering at what point does an atom become radioactive and do all nuclei after that point become splitable?
[Edited on 1-5-2016 by soma]
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Fulmen
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Now you're starting to ask hard questions.
This addresses nuclear stability: http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Nuclear_...
Here is the Wikipedia page on fissile isotopes: https://en.wikipedia.org/wiki/Fissile_material
For all but the heaviest elements the decay is almost exclusively β- (a neutron decaying into a proton and an electron) or β+
(a proton decaying into a neutron and a positron).
As the nucleus grows, the probability of α-decay (ejection of a helium nucleus) increases. As I understand it these decay modes are driven by
different processes. The β-decay is caused by an unfavorable N:Z (neutron/proton) -ratio, while α is caused by the nuclear binding energy.
As the nucleus grows larger than iron the nucleus slowly becomes unstable as it's energy increases, this happens regardless of it's N/Z-ratio.
Ejecting an α-particle allows the nucleus to move closer to iron which is the most stable configuration.
For very heavy elements there are "impossible" configurations of the nucleus, this is again related to the N/Z-ratio (but more complex than a simple
excess of neutrons or protons). Due to the increased energy these will tear them selves apart rather than simply eject an α-particle.
So injecting a neutron into a nucleus that is one neutron shy of an impossible combination will cause the nucleus to split in half, these are called
fissionable.
[Edited on 2-5-16 by Fulmen]
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