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It the notation $\mathrm{p_x}$, $\mathrm{p_y}$ and $\mathrm{d_{x^2{-}y^2}}$, are the subscripts variables (and therefore should be in italics) or are they labels (labels for directions, I suppose, and therefore should be upright)?

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    $\begingroup$ The notation is for the subshells of electrons in atoms. The uses of x,y, and z correspond to the x,y and z axis in 3D space, and are therefore labels not variables. See: en.wikipedia.org/wiki/Atomic_orbital#Orbitals_table for the visualized shapes. $\endgroup$
    – MaxW
    Commented Feb 21, 2017 at 6:36
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    $\begingroup$ @Max you sure they should be upright? I thought they're used slanted-ly everywhere. $\endgroup$
    – M.A.R.
    Commented Feb 21, 2017 at 8:55
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    $\begingroup$ I'll defer to the ACS style guide -- italic. $\endgroup$
    – MaxW
    Commented Feb 21, 2017 at 14:59

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The subscripts that specify orbital axes are written in italic. This is mentioned in ACS style guide.$^{[1]}$ So for instance

$$\mathrm{d}_{x^2 - y^2}.$$

Be careful though. Other subscripts for orbitals are generally upright, e.g.,

$$\mathrm{t_{2g}}.$$

$[1]$ Anne M. Coghill, Lorrin R. Garson. ($2006$). The ACS Style Guide. Effective Communication of Scientific Information. American Chemical Society. DOI: 10.1021/bk-2006-STYG, ISBN: 9780841239999 (print), 9780841228306 (online). (p 256)

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  • $\begingroup$ I would say this contradicts the basic rule of labels being upright (and I consider an axis name a label), but there we go. $\endgroup$
    – mhchem
    Commented Feb 21, 2017 at 9:41
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    $\begingroup$ @mhchem No, I don't think that is correct. It does not refer to the axis. It refers to a mathematical function which appears in the wavefunction corresponding to the orbital. There is no "x2-y2" axis. It just means that that d orbital has a x2-y2 term in its wavefunction. $\endgroup$ Commented Feb 21, 2017 at 10:41
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    $\begingroup$ The p orbitals just happen to be "(a whole bunch of other stuff) multiplied by one of (x, y, z)". The p_z orbital does indeed point along the z-axis but it is not named so because it points along that axis. $\endgroup$ Commented Feb 21, 2017 at 10:42
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    $\begingroup$ Yes, I believe @orthocresol is correct. The terms are still called orbital axes though AFAIK, but the analogy indeed only works for $(x, y, z)$. (Correct me if I am wrong.) $\endgroup$ Commented Feb 21, 2017 at 11:29
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    $\begingroup$ See also: Why do the d orbitals have these notations? $\endgroup$ Commented Feb 21, 2017 at 12:07

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