Will electrons pair up in an orbital only when all orbitals in different sub-levels have one electron?

Usually, yes, but not necessarily.

For a given `n`, different sublevels have differing `l`, where `l = 0, 1, 2, . . . , n-1`.

Hund's rule states that in order to maximize spin multiplicity, an atom usually fills one orbital at a time, and pairs up after every orbital of the same energy (or neighborhood of that energy) is half-filled.

Well, a counterexample is that thorium (`"Th"`) has a `7s` orbital that is pretty much the same energy as its `6d` AND `5f` orbitals (empty circle vs. empty triangle vs. filled diamond):

![http://onlinelibrary.wiley.com/doi/10.1002/9781118688304.ch8/summary, pp. 199 - 202]()

And yet, its electron configuration is:

`color(blue)([Rn]7s^2 6d^2 5f^0)`

rather than... (as lower `n` is usually lower in energy)

`[Rn]7s^2 6d^0 5f^2`
(Aufbau)

or... (as it decreases electron pairing interactions)

`[Rn]7s^1 6d^3 5f^0`
(Hund)

or even more seemingly reasonable... (as lower `n` is usually lower in energy)

`[Rn]7s^1 6d^0 5f^3`
(Aufbau + Hund)

And the reasoning behind that is the `5f` orbitals are particularly compact compared to the `6d` that `"Th"` would rather have `[Rn] 7s^1 6d^2` rather than `[Rn] 7s^1 5f^2`, even though (or rather, BECAUSE) they are all pretty much the same energy.

The `6d` orbitals are also apparently not so diffuse that a `[Rn] 7s^1 6d^3` ends up being viable, so that the `7s` being doubly-occupied leads to the lowest-energy ground-state thorium atom.