Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about atom behavior

  1. Jul 22, 2012 #1
    i've read a theory about dark stars resently and i've got some questions thats been gnawing at my mind, hope i can get some answers here.

    1: what is the size of an atom? as in if the nucleus was the sun, where would the electrons be? i know diffrent substances has diffrent atom sizes but for the sake of argument lets use an atom like helium. so if the helium atomic nucleus was the sun, its electrons would be in what planetary orbit?

    2: i dont know what orbit you choose but say you pick mars for instance, would gravity be able to change the orbit of the electrons by squeezing them in to a closer orbit like say venus?
    is it possible?

    got more questions but they depend heavily on the first two. so if anyone could enlighten me i would really apreciate it.
  2. jcsd
  3. Jul 22, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Hmm. I believe that in an atom such as helium, if the nucleus was the size of the sun, the electron cloud would be about 1/10th of a light year away. That's about 100,000 times the size of the Sun/nucleus. (Very rough calculation) Either way it is far beyond the orbit of any planets in the solar system.

    Let's not take our analogy too far. Electrons do NOT orbit the atomic nucleus like a planet orbits a star. They exist in energy levels and do not have a set place, but a probability cloud.
  4. Jul 22, 2012 #3
    ahh :) ok, thank you for the answer. here are some follow up questions.

    is there a force that can move atoms closer together? for instance if 2 helium atoms (using the solar system analogy) get closer to eachother then 1/10th of a light year. say 1/20th of a lightyear. would the electrons cross the orbits of eachother? seen above like 2 circles crossing into eachother or would the orbits of those electrons shrink in on their home nucleus to avoid collision?
  5. Jul 22, 2012 #4
    i meant clouds not orbits. and if your answer is that the clouds would change shape like 2 sponges pressed against eachother then i would like to know what would happen if you had 7 helium atoms, 1 in the middle 4 on the sides and 1 on top and bottom. what would happen to the middle atom? would there be a force that would stop the surrounding atoms from pushing it in? and if so, is there a force that could overcome that force? like gravity.
  6. Jul 22, 2012 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    Let's drop the solar system analogy and just use the standard models. Electrons are attracted to the nucleus through the EM force. Two helium atoms that get close to each other so that their electron clouds begin to overlap will feel a repulsion as their electrons repel each other. This will cause them to move away from each other if possible.

    This generally does not happen. Helium is an inert atom and has a full outer shell of electrons, causing it to not bond with other atoms. Since it doesn't bond with anything else you can't set it up like this, the helium atoms will just move away from each other when you try.

    When we get down to the atomic scale, things do not behave like they do in the real world. In order for the electron orbitals to change shape the electrons would have to move into different, higher energy orbitals. This requires energy. If two atoms move very slowly towards each other until they start to feel a repulsion this does not cause their clouds to deform, as their is no energy available to cause the electrons to move into higher energy orbitals nor are they bonding with each other.
  7. Jul 22, 2012 #6


    User Avatar
    Gold Member

    A minor point, but I do believe that electrons and atoms exist in the real world, so perhaps a better choice of words here would be "macro world"
  8. Jul 22, 2012 #7


    User Avatar

    Through the magic wardrobe and into Narnia of the quantum world.
  9. Jul 22, 2012 #8


    User Avatar

    Yes they will repel but with just the right amount of force they will bind under Van der Waals force. The charge density is never perfectly even so if two helium atoms get just close enough at the right spot - the repulsion and attraction will be a net zero, holding them in a van der waal bond.

    No. The electrons need a very specific quanta of energy to move up an energy level - this does not mean the electron cloud is rigid and fixed. It isn't.

    No. The clouds will deform when two atoms moves slowly towards each other. The charge density around an atom is not even and it shifts. And charge aside, there are other interesting deformations.

    The electron orbiting the nucleus like a planet is a very bad way to visualise what's happening. And some people would say you should never make any attempt at visualising the atom beyond saying it looks like this; ψ, a devils pitchfork, or candelabra.

    One handy way of visualising atoms and molecules is an electrostatic potential map. You can think of the map representing the electron cloud.

    Here's a picture of Methane and water. The blue, green, and red, represent different charge densities, positive, negative, or neutralish.

  10. Jul 22, 2012 #9


    User Avatar
    Staff Emeritus
    Science Advisor

    I was under the impression that generally it required a transition to a new state in order for there to be any major shift in the orbital shapes. Apparently I was mistaken.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook