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Why atom is structured the way it is?

  1. Jul 25, 2012 #1
    Hello guys - my first post!
    Why do we have neutrons and protons in the center and electrons around them? Was the atom like this since the beginning of the universe(or rather did atom exist in the beginning) or did it get structured in this way after sometime?
    Did the neutrons, electrons and protons exist independently or were they always together like we know today?
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  3. Jul 25, 2012 #2


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    Hi, Avichal!
    Others are far more knowledgeable than me, but a couple of pointers:

    The "atomic structuring" of our universe can be regarded as the natural outcome of the physical laws, once the universe has cooled and expanded sufficiently.

    In remaining hotspots of the universe, as in the hottest regions of a star, the atomic structuring of matter is NOT what is found; instead, what you have there is what is called the plasma state of matter.
    A plasma can be regarded as a soup of free-floating protons, electrons that etc. are NOT tied together as atoms.
  4. Jul 25, 2012 #3
    Neutrons and protons are both baryons, and are made of particles called quarks. Quarks interact with each other through the strong nuclear force, which is mediated by the gluons. There are 6 total flavors of quarks, 3 if which have charge 2/3, and 3 that have charge -1/3. The lightest quarks, and therefore the ones that make up neutrons and protons, are the up and down quark. Each quark flavor also has three different colors, red, green and blue. Neutrons and protons are also bound by the strong force. Nuclei formed relatively quickly after the big bang.

    Since electrons are negatively charged (compared to the positively charged protons in the nucleus), they are attracted towards the nucleus. However, the Pauli exclusion principle forbids electrons from occupying the same state. So, the electrons occupy well-defined orbits around the nucleus. That is, their angular momentum is quantized. Electrons were first joined with the nucleus to form atoms 380,000 years after the big bang at the recombination, when the temperature was low enough to allow electrons to become bound to nuclei (about 3000 degrees Kelvin).
  5. Jul 25, 2012 #4


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    Protons and neutrons are heavier than electrons and they interact via the strong interaction. This leads to the structure of atoms. As the physics was (probably) the same at all time, even the first atoms, ~100,000 years after the big bang, looked like this.

    Protons, neutrons and electrons can exist as free particles ("independently"). Free neutrons decay into proton+electron+electronneutrino after a while, the other two particles are (probably) stable.
  6. Jul 25, 2012 #5
    Well I have no knowledge of quarks and baryons but as far as I infer from you guys is that the structure of atom is the consequence of the laws of physics. Am I right to compare it with solar systems - The more massive object like star stays in the center while the other small objects revolve around it?
  7. Jul 25, 2012 #6
    Not really. Solar systems work that way because of gravity. Gravity is irrelevant inside of the atom. It just happens that the nucleus is heavier because of the contribution of mass from the binding energy that holds quarks together.

    Also, it isn't correct to think of electrons as orbiting the nucleus. Because of Heisenberg's uncertainty principle, the electron doesn't have a well-defined position. You can only specify the probability that the electron will be in a certain place around the nucleus.
  8. Jul 25, 2012 #7


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    Well, if it was light (similar to electrons), we would have a different shape of atoms.

    The structure (heavy thing in the center, lighter things around it) is similar to the solar system, and that is not by coincidence. However, unlike planets, electrons do not have well-defined orbits.
  9. Jul 25, 2012 #8
    There are two differences as I see it between the atom and the solar system - (1) the governing force is electrostatic, rather than gravitational and (2) the framework we use to analyse the motions is quantum mechanics, not classical mechanics*. So we have lots of differences arising, for example, the electrons mutually repel, which doesn't happen with planets, and electrons occupy orbital states with well-defined energies, rather than planets, which can have any energy if you just give it the right kick (and by "any", I mean "any with respect to some pre-defined zero level").

    *at an undergrad level, without going into general relativity (planets) or relativistic/higher order EM corrections (atom).

    I just realised I missed the neutrons/protons part. For the above I just assumed they were stuck together in a nuclei.
  10. Jul 25, 2012 #9
    Yes I know that atom and solar system have differences. But the fact that neutrons and protons stay in the center while electrons move around is because of their respective masses right?
    Also do we know what matter was like in the beginning of the universe? Was there a sea of protons , electrons and other elementary particles or was the atom(hydrogen atom) already structured?
  11. Jul 25, 2012 #10
    We are talking about very different scales at the same time which might cause confusion:

    On the atomic scale we have electrons and nucleus acting under the Coulomb force. To answer your first question, the nucleus contains the vast majority of the total mass in itself (as with the sun). It is a simple conclusion that the centre-of-mass be nearby. This explanation doesn't require any knowledge of the sub-nuclear particles.

    On the nuclear scale we have protons and neutrons bound together according to some complicated rules, specifically the Coulomb force and the strong force. The binding of nuclei was taught to me using the semi-empirical-mass-formula which describes how the binding strength varies for different proton/neutron combinations.

    On the hadron scale we have different quarks being under the influence almost exclusively of the strong force.

    Regarding your second question, most atoms formed at recombination 300,000 years after the big bang, most nuclei formed at nucleosynthesis about 3 minutes after the big bang, and I don't really know if or when most hadrons formed, although I did find a wikipedia article on "baryogenesis" which might help.
  12. Jul 25, 2012 #11
    Ok, thank you all
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