Atomic structure
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I am very confused about the order of filling, and energy of atomic energy levels. Why does the 4s sub-shell fill before the 3d sub-shell? Why are they labelled 4 and 3 if they fill in this way? Please put me straight.
Energy levels in a hydrogen atom
It helps to start by looking at the energy levels in a hydrogen atom. A hydrogen atom has just one electron which is usually in the lowest 1s sub-shell. This is an atomic orbital.
The pattern of energy levels is relatively simple because in a hydrogen atom all the sub-shells in each main shell have the same energy. In other words the 3s, 3p and 3d sub-shells all have the same energy in a hydrogen atom.
In a hydrogen atom the higher energy levels are closer together than the lower levels. The gap between the 3rd shell (and all its sub-shells) and the 4th shell (with its sub-shells is quite small).
Energy levels in atoms with more than one electron
Once atoms contain more than one electron, the interactions between electrons mean that the sub-shells in a main shell no longer have the same energy. This is a consequence of the different shapes of the atomic orbitals corresponding to each sub-shell (for example, s-orbitals are spherical while p-orbitals are dumbell shaped)
In an atom with several electrons it turns out that for the group of sub-shells labelled 3s, 3p and 3d, the 3s orbital has the lowest energy and the 3d the highest. We know that they should all be numbered '3' because they can be traced back to the third main shell in a hydrogen atom.
The widening gap between sub-shells in a main shell means that by the time we get to potassium and calcium, with lots of electrons, the 4s sub-shell has a slightly lower energy than the 3d sub-shells and so it fills first. The rule being always that the lowest sub-shell with space for an electron fills first.
However next in line are the 3d sub-shells which fill before the 4p.
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This article is the most helpful aid to my Chemistry studies so far- it is masterfully written; and explains a question which tutors frequently fail to address. Thank you very much.
Catherine 16 October 2004
updated: 22 December 2003
