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Unintuituve naming convention: para- denotes antiparallell?

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Apr3-09, 11:37 AM
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I did a project a week ago that required studies of the helium atom. Depending on if the two electrons spin are parallell or anti-parallell it is referred to as orthohelium (parallell) or parahelium (antiparallell).

Intuitively I would have imagined the naming convention the other way round: parahelium for parallell spins.

Today I "corrected" a professor in subatomic physics when positronium came up which apparently has the same naming convention. My thoughts are along the line: "if even HE finds it unintuitive, why is it so?" :-) Is it something else that is parallell/orthogonal, or is it just historic reasons?

Google didn't help me.
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Apr3-09, 12:44 PM
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f95toli's Avatar
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It is for historical reasons. Para means "next to" or "beyond" in latin (and ortho means straight), as in paranormal.
It is presumably because this was orignally the names of lines in the spectrum of helium There are lots of other examples of this, the "d" in e.g. 2d orbitals is short for "diffuse" (as in a diffuse line on the photographic plate used to capture the spectrum).
Apr3-09, 02:10 PM
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It's also just a historic thing for helium. The general terms are singlet/triplet.
In fact, I'd recommend against using the 'ortho/para helium' terminology; it can only cause confusion really.

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