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Author Topic: What determines the numbers of protons and neutrons in an atom?  (Read 8565 times)

John Law

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John Law asked the Naked Scientists:

We all know that in a neutral atom there is exactly the same whole number of electrons and protons. Charge on an atom is always a whole number positive or negative.

So how does the atom keep account? Surely the only possible way is if every proton wants to pair to exactly one electron. Could this be a window into the nucleus?

What do you think?


 

Offline stevewillie

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The structure of the universe would seem to have been encoded at the beginning, that is when time began with the Big Bang. No one, at least now, can know what happened before the point when the universe was at Planck dimensions. (about 10^-35 meters and 10^-43 seconds. All physical theory breaks down at this point. It's not known what happened "immediately" after this point either, but in theory it could be known if we had sufficiently powerful colliders. In an earlier post I gave a hypothetical example of a collider that would fill the orbit of Neptune. I don't know how close it would get us to the Planck dimension, but it wouldn't get us to it and it can't us beyond it. Some argue that the Big Bang isn't a singularity, but a very small "tube" that links us to a multiverse. It's way beyond our technology to test this, so it remains just an idea.

Unless you posit an unseen hand from outside the universe guiding its evolution and making "choices", I think you have to assume its structure was encoded at the beginning. Something begat quarks and leptons. The quarks begat protons, some of which mated with electrons (a lepton) to make neutrons. The mutual attraction of protons and electrons begat the hydrogen atoms which condensed to make stars. (Since hydrogen atoms contain no neutrons, its possible that neutrons were created later in the stars). The stars created the nuclei of the heavier elements by the fusion process,which then attracted their electron shells as they were ejected from the stars. Someone else will need to explain just why the elements have varying numbers of neutrons. Hydrogen can have up to two to go with its single proton. For some reason, the number of neutrons equals the number of protons in the nucleus of the most stable atoms (except hydrogen). Many isotopes exist, where there are extra neutrons. Stability seems to be the limiting factors. For instance, 'basic' carbon has six protons and six neutrons. This is the stable carbon 12. There are two common isotopes: carbon 13 and carbon 14 which have one and two extra neutrons respectively. The latter is the least stable, and its half-life is the basis for carbon dating. I hope I hit this right, neither too simplistic, nor too difficult. To fully answer your question requires a knowledge of nuclear physics which I can't claim to have.     
 

Offline DrDick

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The charge is a whole number because we say it is.  The +1, -2, or whatever refers to the polarity and the number of net electronic charges present.  Charge is actually measured in coulombs, and an electron or proton has a charge of -/+ 1.60 x 10-19 coulombs.  This is the smallest increment of charge that we can measure (supposedly the smallest increment that exists, but who knows what subatomic particles we may discover in the future).  If the charge is +3.20 x 10-19, then we say the ion has a charge of +2.

As far as the atom "keeping track", it's actually quite simple.  It doesn't.  Electrons can adhere to an existing atom, creating an anion.  They can leave an existing atom, creating a cation.  The tendency towards this happening depends on electrostatics.  As an electron approaches an atom, it can feel repulsions caused by the electrons already present, and attractions caused by the protons present. 

If the attractive forces are stronger, as tends to be the case with the nonmetals (they're smaller, so the electron can get closer to the nucleus), the electron will adhere to the atom.  If the repulsive forces are stronger, the electron will tend to veer away.

Sometimes, an atom with strong attractive forces approaches an atom with weak attractive forces.  When this occurs, an electron will leave the weakly attractive atom and hop over to the strongly attractive atom.

Now, to comment on something that stevewillie said, most elements do not have an equal number of protons and neutrons.  That's really only partially true, even for elements from helium to calcium (less than a quarter of the stable elements).  After that (and even some elements before that), the atoms prefer to have more neutrons than protons.  By the time you get to bismuth (the last element with a stable isotope), the ratio of neutrons to protons is about 1.5 : 1.
 

Offline stevewillie

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I
The charge is a whole number because we say it is.  The +1, -2, or whatever refers to the polarity and the number of net electronic charges present.  Charge is actually measured in coulombs, and an electron or proton has a charge of -/+ 1.60 x 10-19 coulombs.  This is the smallest increment of charge that we can measure (supposedly the smallest increment that exists, but who knows what subatomic particles we may discover in the future).  If the charge is +3.20 x 10-19, then we say the ion has a charge of +2.

As far as the atom "keeping track", it's actually quite simple.  It doesn't.  Electrons can adhere to an existing atom, creating an anion.  They can leave an existing atom, creating a cation.  The tendency towards this happening depends on electrostatics.  As an electron approaches an atom, it can feel repulsions caused by the electrons already present, and attractions caused by the protons present. 

If the attractive forces are stronger, as tends to be the case with the nonmetals (they're smaller, so the electron can get closer to the nucleus), the electron will adhere to the atom.  If the repulsive forces are stronger, the electron will tend to veer away.

Sometimes, an atom with strong attractive forces approaches an atom with weak attractive forces.  When this occurs, an electron will leave the weakly attractive atom and hop over to the strongly attractive atom.

Now, to comment on something that stevewillie said, most elements do not have an equal number of protons and neutrons.  That's really only partially true, even for elements from helium to calcium (less than a quarter of the stable elements).  After that (and even some elements before that), the atoms prefer to have more neutrons than protons.  By the time you get to bismuth (the last element with a stable isotope), the ratio of neutrons to protons is about 1.5 : 1.

I pulled this post from the "Questions Needing Answers" file. There are perfectly good questions there that don't get responses, so I gave it a shot. I'm neither a chemist nor a physicist. My estimation was that John Law understood that positive and negative charges must balance in an intact atom, so I thought I'd give a brief description of cosmic evolution up to the point of the hydrogen atoms, indicating that atoms are what they are as result of the way atom's "ancestors" combine with rising entropy. I focused more on the neutrons because it's not clear (to me at least) just why atoms have the number neutrons they do. Its odd that hydrogen can have an isotope with a mass number three times H1. Carbon's heaviest commom isotope is 7/6 the mass number of C12. The weak force is apparently responsible for keeping the neutrons in the nucleus and its carrier particle has been found. However the strong force carrier has not,to my knowledge, been found. I don't know the anything about the interaction between protons and neutrons within an intact atomic nucleus. Can you shed some light on this?
 

Offline DrDick

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I honestly don't know that much about nuclear physics/nuclear chemistry.  I kind of look at neutrons as being a buffer needed to shield the positive charges of the protons from each other. 

Hydrogen, with only one proton, doesn't need this buffer, so it's hard to find any atoms with any neutrons at all.  As soon as you add a second proton (helium), you need to add that buffer, so neutrons are necessary.  I know that there are certain "magic numbers" with respect to proton and neutron count, but I have no knowledge of the basis of those numbers.

I see most of chemistry arising from the number of electrons present, so I tend to look at all chemical problems from this point of view.  Of course, the number of electrons is intimately dependent on the number of protons, which is why we tend to organize the periodic table by atomic number.

I didn't focus much on neutrons because it didn't appear to have anything to do with the original question.

Dick
 

Offline stevewillie

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Thanks for the responses. We got John Law's question answered. I haven't figured out why some questions which I regard as silly and non-scientific get substanial responses while to-the-point scientific questions end up in limbo. (Actually I have figured it out, but I don't like the answer.)
 

Offline DrDick

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Thanks for the responses. We got John Law's question answered. I haven't figured out why some questions which I regard as silly and non-scientific get substanial responses while to-the-point scientific questions end up in limbo. (Actually I have figured it out, but I don't like the answer.)

So what's the answer?
 

paul.fr

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Thanks for the responses. We got John Law's question answered. I haven't figured out why some questions which I regard as silly and non-scientific get substanial responses while to-the-point scientific questions end up in limbo. (Actually I have figured it out, but I don't like the answer.)

So what's the answer?

I hope it's not 42. But I too would like to know the answer.
 

Offline stevewillie

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My take is that a lot of people think that to-the-point scientific questions are boring while silly questions allow a lot of latitude for banter. That's OK for some of the forums like "That Can't Be True" but this was a chemistry question that deserved a response from someone. Paul, I don't know what you mean by '42'.

If you're talking about Dr Dick's and my responses to John Law's question, I probably should have said 'responded', rather than 'answered'. I frankly don't why atoms have the number of neutrons they do. Do you? The number of protons fills the range from 1 to 92 for stable natural atoms. So nature simply has filled the sequence up to the apparent limit of stability. 
« Last Edit: 24/09/2008 12:08:55 by stevewillie »
 

Offline rosy

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It's really not a chemistry question, tho' it's been put in this thread by one of the TNS team. It's a (particle) physics question.

On the subject of which questions get answered... I think you're oversimplifying quite a lot, actually.

The reason lots of good questions don't get answers and lots of fatuous questions do is because of the effort required to produce the coherent and informative response to a good question at an appropriate level.

This question has (eventually) received a good response (tho' I think it's actually a question asked by a listener to the show and posted after the fact by the TNS team), but many such questions which don't really have straightforward answers (or not without recourse to an extended discussion of the strong and weak forces) are left unanswered because they're just too difficult.

Also, quite a lot of the "silly" questions are either (1) asked by regular forum members and less silly than they appear at first sight or (2) asked by, not to put too fine a point on it, nutters. Both of these (sometimes overlapping) groups tend to add comments to a thread if they don't get a response, which keeps them at the top of the recent posts list and thus makes them more likely to be encountered by someone who can provide a reasonable answer.

I personally don't often bother pulling questions out of the QTNA list because I'd sooner have an audience if I'm going to go to the trouble of answering a question, and by and large if something is in the QTNA section unless the person who asked it is a forum regular they've probably given up expecting a reply and won't get to read it even if I post one.

And.. Paul's reference to 42 was presumably to the "answer to life the universe and everything" as per Douglas Adams "The Hitch-Hiker's Guide to the Galaxy".
 

paul.fr

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What determines the numbers of protons and neutrons in an atom?
« Reply #10 on: 24/09/2008 13:28:41 »
My take is that a lot of people think that to-the-point scientific questions are boring while silly questions allow a lot of latitude for banter. That's OK for some of the forums like "That Can't Be True" but this was a chemistry question that deserved a response from someone.

To add to what Rosy has already said, in reply to the above.

Yes, it is just too simple a view on how the forum works and the way questions are answered. As you say, this question was picked from the QTNA section, I would say it was there because nobody, at the time the question was posted, had an answer. Not every member logs in daily, not every member will check all the different boards, and not every member will be able to answer such a question, as a result they do slip down the listings and just may end up gathering dust until someone (normally a new member) goes looking the the lists of previous posts.

This does not make the question boring, it may just mean that a member capable of answering such a question has not logged in or simply did not have the time to write a comprehensive reply.

As for the silly questions. These are the lifeblood of the forum, they may look silly but are infact drafted in such a way that they will appeal to members who would not normally post in the more sciency area's. They allow people to "have a go" at answering the question without feeling that their answer is silly or makes them look stupid, yet, they are also answered by members who are trained in a science discipline. These type of questions help a broad spectrum of members participate in the forum, if all we had were the straight to the point "heavy" science questions then this would not be The Naked Scientists forum.

To me, the forum is one that is based on science, but heavily laced with fun and humor. Not everyone understands all of the humor, but the appeal of this forum is that people from all backgrounds can happily participate. Yes we have our fair share of "nutters", and their topics do tend to be the ones with more that their fair share of replies, but that is because some members will give their time to put those posts and posters straight, afterall, people may read something false here and take that away with them as a truth.
 

Offline stevewillie

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What determines the numbers of protons and neutrons in an atom?
« Reply #11 on: 24/09/2008 19:59:08 »
OK. But there are plenty of forums for "fun" (actually I consider science "fun"). I guess what I dislike is that you have forums for chemistry, physics and cosmology, etc and reasonably serious questions on these kinds of sites probably should be responded to. However, as you say,it is often the nutty questions on "hard" science forums that often get double digit responses from members. I've gotten a few "I don't know but...." answers and I can accept that. But I also withdrew some questions that didn't get a response after a few days. Actually this is a good feature of TNS. Your unloved question doesn't drop down on an ever lengthening list as it does in other forums.
« Last Edit: 24/09/2008 20:06:16 by stevewillie »
 

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What determines the numbers of protons and neutrons in an atom?
« Reply #11 on: 24/09/2008 19:59:08 »

 

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