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Why can Period 3 only contain upto 8 electrons, not 18?
Period 3 contains 8 electrons, but the 3d sub-shell adds another 10 to Period 4 not Period 3, why is this? This fact has bugged me since we learnt about electron structures in Senior School. It would be nice to finally resolve the issue, as neither me nor my friends can adequately say. The reason Period 3 does not fill out its maximum occupancy of 18 electrons is that the 3d subshell turns out to be somewhat higher in energy than the 4s subshell. And why is that? "Just because." The problem is that, while it's a useful model, the aufbau principle (in which hydrogen-like atomic orbitals are filled two electrons at a time) is not strictly correct. For one thing, we cannot observe orbital occupancy directly; instead we observe the total spectroscopic state of an atom. The orbital occupancies published on periodic tables are obtained by fitting the spectral data to a hydrogen-like model. And is the hydrogen-like model valid? We don't know. You see, we can't solve any problem with more than two bodies (and that's any atom from helium on up) analytically. What we do is algorithmically approximate the solution by modifying the results from a problem we can solve until they converge, that is, minor modifications no longer produce significant changes in the result. In atomic physics, we use the solution to the hydrogen atom as a starting point for many-electron atoms. This approach is also taken in astronomy and astrophysics. Gravitational perturbations of a satellite's orbit are considered as a sequence of two-body interactions until the result converges.While we can produce reasonable explanations of spectroscopic data in terms of electrons occupying hydrogen-like atomic orbitals, we can't really explain why orbitals are filled in the order they are except by "hand-waving" explanations. I will try to supply one now: Ahem. S atomic orbitals are allowed to approach the nucleus more closely than p or d or f atomic orbitals. The distance an electron is constrained to stay away from the nucleus increases with the azimuthal quantum number (which defines the type of orbital), so that p electrons are more closely-held than d electrons. Since the closer an electron can get to the nucleus, the lower its energy, it turns out that (since 4s electrons can get a little closer than 3d electrons) the 4s orbital -- when empty -- is a little lower in energy than an empty 3d. In other words, the potassium atom as a whole is a little lower in energy with a [Ne]3s23p64s1 configuration than with a [Ne]3s23p63d1 configuration. For a more detailed explanation, see
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