• Question: Is there a maximum of shells in an atom? Or not, I don't think there is but I'm not too sure.

    Asked by bethl to Ben, Jony, Katharine, Mark, Peter on 15 Nov 2011.
    • Photo: Jony Hudson

      Jony Hudson answered on 15 Nov 2011:

      So, theoretically there are an infinite number of possible shells, because they get closer and closer together as you go up in energy.

      But actually, electrons in these very high energy shells (called Rydberg atoms – google it!) are so weakly attached to the nucleus that even the tiniest nudge will knock them off the atom. So practically speaking, in nature, there’s a limit to how you can go.

    • Photo: Ben Still

      Ben Still answered on 15 Nov 2011:

      No maximum in theory but in practice there are limits to which the forces can hold the electrons to the nucleus of an atom and indeed the protons in the nucleus. The number of protons in the nucleus determines the number of electrons that can be attracted to orbit. This plus the fact that as you go to higher orbits the electrons fell less and less attraction from the nucleus leads to a two fold effect and cuts of the size of atoms.

    • Photo: Mark Basham

      Mark Basham answered on 15 Nov 2011:

      Hi bethl,

      Ben and Jony have this one wrapped up, don’t think there’s any more to add.

    • Photo: Katharine Schofield

      Katharine Schofield answered on 15 Nov 2011:

      Well, I think I can add to what Jony and Ben said, which was all about electron shells. The reason that electrons arrange themselves into shells can all be explained by quantum physics. But it’s not just the electrons in an atom that arrange themselves into shells…the protons and neutrons in the nucleus do too. Nuclei are quantum objects, and although the maths is messier than with electrons, the shell model of nuclei has helped nuclear physicists explain why some nuclei are stable and some are radioactive (i.e. they decay, emitting radiation). So if the outermost neutron and proton shells are full, the nucleus sits quite happily and doesn’t decay (a bit like atoms with full electron shells not being very reactive with other atoms). If the outer shells aren’t full, the nucleus prefers to get rid of these hangers-on and so is more likely to decay.

      Hope I haven’t confused you! I didn’t learn about this until uni… And we still don’t have a complete explanation of how all nuclei work from the lightest to the heaviest elements, but there are nuclear physicists out there who are trying!

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