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In postulates of Bohr's model of the atom, it is said that the nucleus is at the centre of the atom. In Rutherford’s α scattering experiment he said that out of 20,000 alpha particles only 1 particle is deviated by $180^\circ$. If the particles in the atom are moving continuously (like an electron), why is that nucleus at the centre of the atom? If that is so, then when we focus all the α particles at the nucleus (the point where an α particle got deviated by $180^\circ$) all should get deviated by $180^\circ$ right?

What I mean is that why is the nucleus at the centre of the atom?

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    $\begingroup$ Forget the Bohr's model. The particles in the atom are not really moving; also, they are not really particles. $\endgroup$ – Ivan Neretin Feb 12 '16 at 7:33
  • $\begingroup$ I am a 12th class student. Can you please explain? $\endgroup$ – O K RITVIK Feb 12 '16 at 7:34
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    $\begingroup$ Read this, for example: chemistry.stackexchange.com/questions/36987/… $\endgroup$ – Ivan Neretin Feb 12 '16 at 7:45
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    $\begingroup$ @IvanNeretin I have a feeling you read too much quantum stuff ;) At least from OP's point of view. From very basic pov it's similar situation as with Solar System. Nucleus is highly charged and massive analogously to Sun. $\endgroup$ – Mithoron Feb 12 '16 at 17:38
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    $\begingroup$ First, Rutherford did not show that the nucleus was at the center of an atom, only that the majority of the mass of an atom was concentrated into a small volume. Now, given a small nucleus with all the positive charge, the electrons will, to first order, groups around the nucleus, not sit off to one side. For that last bit - how, exactly, do you plan to focus all (or any!) alpha particles so that they will all hit one nucleus? For historical accuracy, the original experiment used alpha particles from natural decay processes, which were collimated with apertures - no focusing. $\endgroup$ – Jon Custer Feb 12 '16 at 20:06
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Why is the nucleus the centre of the atom?

  1. Definition. Whatever the electron does, chemists define the position of the nucleus to be the centre of the atom meaning that electrons can be polarised away.

  2. Quantum mechanics. When studying an isolated atom, the only thing that influences electron behaviour is the nucleus. You can calculate the ‘path’ of an atom relative to the position of the nucleus with quantum mechanics and your result will be that quite a few electrons will occupy a perfect sphere whose centre is the nucleus.

The second point may require elaboration. But it really is quite simple. You have a very lightweight, very fast particle with a charge of $-1$ (your electron) and you have a very heavy, very slow particle with a charge of $+1$ (your proton). By the time the proton has moved an inch, the electron has moved a mile so we can approximate that only the electron really moves. (This has the fancy name Born-Oppenheimer approximation.) Now remember that both are attracted to each other. The proton doesn’t move, but the electron feels the ‘desire’ to ‘move towards’ the proton. (Inverted commas because this is a macroscopic view that only gets us so far when dealing with quantum effects.) If you average this over time, you will find that the electron will have spent the same amount of time on one side of the nucleus as on the other and the mean electron position will be in the nucleus. Spherically outwards from the proton, the chance of locating the atom decreases monotonously with distance; the angle does not matter. We have a perfect sphere with the proton being the centre.

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There are so many different approaches to this question, I am sure. But, if you look at it strictly from a very simplistic perspective, the nucleus has a net positive charge and the electron cloud has a net negative charge. The electron cloud gets as close as it can to the nucleus, but not too close due to inter-electron repulsion. That makes the most probable distribution of the electron cloud to be a uniform distribution with the nucleus at the center. Because the position of the nucleus is only a probability, not all focused alpha particles would back-scatter by 180 degrees experimentally.

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