1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Here is an excerpt i found on the web. Ernest Rutherford, born on

  1. Feb 7, 2012 #1
    here is an excerpt i found on the web.

    Ernest Rutherford, born on New Zealand, showed in 1911 that alpha particles were sometimes very strongly scattered by the positive charges of the atom, in a way that could only be explained if such charges were concentrated in a very small volume, practically a point in space. He therefore suggested that every atom had a compact nucleus, with negative electrons floating around it.

    Rutherford's picture suggested that the nucleus was like a miniature Sun, with electrons orbiting it like planets. If Newton's laws were valid on the atomic scale, that might indeed be so, but as later research showed, on the atomic distance scale Newton's laws change into other forms. By these new laws of "quantum mechanics," electrons do not move in precisely defined orbits, but are distributed in space in a way that only allows the probability of finding them anywhere to be calculated. Similarly, energized atoms are only allowed to exist in one of a number of energy levels
    http://www-spof.gsfc.nasa.gov/stargaze/L… [Broken]


    Ok, so basically all of science resides on this dudes theory that atoms MAY have a compact nucleus and at the time of 1911, we had NO WAY to see if it was true? How do we know that his theory is true? are there actual real images of atoms containing a nucleus? Or is it still just a theory and maybe we are all wrong!


    I just need to know because to me science is soo random that it drives me nuts. It does not explain the whole picture to me. its like a piece of sand judging the whole world based off of where its at. All it sees is other pieces of sand, while there are oceans, trees and animals.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Feb 7, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Re: rutherford

    Please start by read the FAQ sub-forum in the General Physics forum.

    If since is "soo random", how in the world did your computer work to allow you to post that?

  4. Feb 7, 2012 #3


    User Avatar

    Staff: Mentor

    Re: rutherford

    First, just because it's a pet peeve of mine, saying something is "just a theory" just shows your lack of knowledge about science. (Not criticizing, just an observation)A scientific theory is NOT what most people think of when they think of a theory. See here: http://en.wikipedia.org/wiki/Scientific_theory

    Second, firing alpha particles at the gold sheet IS how we "saw" it was true. You cannot simply shine light on an atom and get a picture like you do with your camera at home taking pictures of everyday objects and people. Atoms are so very tiny that they are actually smaller than the light is. The reason it's like this is fairly complicated and would require explaining several different concepts that you may not know anything about, but if you would like to know then just say so and I will find some good links.

    Third, unless we wanted to throw out most of physics the ONLY possible reason for it to bounce back was if most of the atom was in a very small dense point of positive charge.

    I don't know what you mean here. What about it is random? And what has science not explained other than the usual questions of "where did the universe come from, why are we here, etc". Keep in mind that unless we can "observe" it (which doesn't just mean "see", but also to measure and interact with) then we can't do much, if any science on it.
  5. Feb 7, 2012 #4
    Re: rutherford

    But we choose unfortunate terms like "string theory" when we only have an hypothesis, and "grand unified theory" even though it is only hypothesised that we can ever find it! It is no wonder the layman doesn't always understand what we say, and understandable that he sometimes decides that we don't know what we are talking about.

    I think we're on fairly safe ground with the compact nucleus though - it may have only been an hypothesis in 1911 but we now have plenty of observation and confirmed prediction.
  6. Feb 7, 2012 #5
    Re: rutherford

    Naa, all of science isn't based on that observation don't worry.

    As for why we believe that atoms have a compact nucleus, there is a tremendous body of experimental and theoretical evidence to support it and nothing to say that it is wrong.

    All it would take, is one experiment that said that it was not compact and we would have to look at things again.
  7. Feb 7, 2012 #6


    User Avatar

    Staff: Mentor

    Re: rutherford

    I'd bet that string theory and such aren't the reason why many people don't believe we have any idea what we are talking about. As wikipedia states:

    The term "theory" is a polyseme, even among scientists. While most scientists reserve the term for verifiable principles, others use it to refer to hypothetical frameworks.

    It is probably unfortunate that they are referred to as theories, but I'd guess that the real reason is usually that much of science just doesn't make any sense to most people and the term "theory" is taken completely out of context.
  8. Feb 7, 2012 #7


    User Avatar

    Staff: Mentor

    Re: rutherford

    Yes, of course the nuclear theory is still "just a theory".... Sort of like how it's "just a theory" that when I put a pot of water on the stove and turn the burner on, it's the heat from the flame that makes the water boil.... That's "just a theory" too. The heat-boils-water theory is a really good theory, it's easy to find a lot of people who believe it, it would be totally amazing for it to be wrong, the theory is reconfirmed hundreds of millions of times a day in kitchens across the world. But it's still "just a theory", and maybe we are all wrong.

    So of course the nuclear theory is still "just a theory". Suppose someone were able to come up with a different theory that:
    1) Explained everything that the nuclear theory explains, as least as well. That's a really tough challenge. This new theory would have calculate the expected deflections and angles of Rutherford's alpha particles; explain mathematically atom bombs and nuclear reactors; the behavior of bazillions of different elements when bombarded with various particles; and countless other things that we've observed over the last century.
    2) Explained at least one thing that the nuclear theory didn't.

    If someone were able to find such a new theory, we could point to it and say "See? We were wrong and this new theory is a 'better' one". The popular press would run around saying that the nuclear theory has been "disproven", "overthrown", shown to be "wrong". Serious scientists would point to #1 above and say that we haven't "disproven" the nuclear theory, we've extended it.

    Don't be so quick to give up on those grains of sand. If you put enough of them together you get a beach... A warm tropical beach with gentle surf, palm fronds rustling in the breeze, sun setting off to the west, cold beer bottle sweating on the table next to you, seabirds and dolphins frolicking in the waves.
    Last edited: Feb 7, 2012
  9. Feb 8, 2012 #8
    Re: rutherford

    I totally agree. thats why i came here. I dont expect sarcastic remarks from you guys because i know you will answer my questions. I do have a lack of science but thats why im in this class to better understand how it works. It just seems odd that they can conclude that just because the light goes through the foil somehow means it has a nucleus. If the nucleus is as dense as they say it is, should it not tear microscopic holes in the foil? Please dont flame me on that question, im seriously asking it to understand. haha.

    Ok so what part of the gold foil exp proved that atoms have a nucleus, the particles bouncing back or the fact that 95% or so made it through? If it is the 95%, then how does that mean that the atom is mostly empty space but contains a nucleus that has protons and netrons? maybe the neutrons are outside floating around with the electrons.

    Drakkith, thank you for you response as well. you are correct, i am lacking in the knowledge, hence why i am here to get answers from the pros. So im just trying to get a bigger picture of how rutherford figured it all out. If you would like to put those links that would be cool. To me, seeing a light shine through gold foil would mean that somehow atoms can fit through a solid with out destroying it. maybe they teleport. haha. they some how make it to the otherside. But how come when i shine a laser on gold foil, it reflects back and none of it passes through?

    thanks guys.
  10. Feb 8, 2012 #9
    Re: rutherford

    In the experiment, they shot alpha particles (essentially a helium nucleolus with the electrons stripped away), not light, at a sheet of gold foil. Alpha particles have a net positive charge, because they consist of 2 protons and 2 neutrons.

    Before the experiment, they actually thought that atoms were just blobs, without any concentrations or empty space. Because of this, they expected that the alpha particles would pass through the gold foil with no issue since the positive charge in the atoms would not be concentrated enough to seriously deflect the alpha particles.

    However, when the did the experiment they found that some particles were being deflected right back at them and all over the room! This meant that there must be some concentration of positive charge (like charges repel) somewhere in the atom.

    They didn't exactly know that atoms were mostly empty space, they just knew that there must be some concentration of positive charge. So the positive bits of the atom were concentrated.

    Also, since the alpha particles were either strongly deflected or were not effected at all, they concluded that there must be mostly empty space in an atom.

    The reason light (a lazer or a flashlight) dose not shine through is because light is deflected by electrons, which are not bound to a small ball in the center of the atom, but rather move around in a cloud orbiting the atom. Whereas alpha particles would only be strongly deflected by 'hitting' the nucleus.

    I also want to reiterate this was just the beginning of many, many more experiments which proved this idea in stronger ways. The body of theoretical physics that was developed years later, quantum electro and chromodynamics, helped us complete our understanding of how atoms worked and why there must be some tight ball in the center with orbiting electrons.
  11. Feb 8, 2012 #10


    User Avatar

    Staff: Mentor

    Re: rutherford

    The atoms in the gold foil are bound to each other pretty tightly. In addition, gold's only stable isotope has an atomic weight of 197 while an alpha particle has an atomic weight of 4. Hence the gold nucleus is almost 50 times more massive, meaning that it won't move much when hit by something light such as the alpha particle.

    At this point in time nothing was known about neutrons. All that was known was that some of the alpha particles were bouncing backwards from the foil. The only way for 95% of the alpha particle to make it through, yet a small percent to bounce backwards had to be a small concentrated mass with positive charge. Think of it like this. The original model was similar to a "fog" or "cloud" with the electrons and protons all spread out within. This spreads the charge and mass out too much for any alpha particles to bounce backwards, they would all pass through with very little deflection. In order to explain this effect, the simplest and easiest explanation that still followed the laws of physics known back then was to model it as a small dense nucleus of positive charge.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook