Why is the electronegativity of hydrogen 2.20?
Interestingly, the first paper from Pauling on electronegativity "The Nature of the Chemical Bond. IV. The Energy of Single Bonds and The Relative Electronegativity Of Atoms" published in 1932 gives a different scale with H = 0.0 and F = 2.0!

Later, he changed to 2.1 for H and 4.0 for F, e.g. http://scarc.library.oregonstate.edu/coll/pauling/bond/notes/sci5.001.14-notes-01.html

If we look through his book, on p. 90 we find the following text:


  Following these there are given the values obtained by adding 2.05 to them...


The resulting numbers are then rounded to one decimal, from H = 2.05 to H = 2.1 and F = 4.0.

His notes are online (e.g., here) but there's no obvious explanation I can find yet.

My hunch is that he started with H = 0.0, and realized that the alkali metals would be negative numbers. The shifted scale gave Li ~ 1.0 and F ~ 4.0, which seem like "nice" numbers.

I tried to hunt down the reason for the move to 2.2 as the standard for hydrogen. The first reference seems to be by Huggins (J. Am. Chem. Soc., 1953, 75 (17), pp 4123–4126) but the paper itself doesn't really explain the reasoning.

Wikipedia suggests a paper by Allred, but that cites the Huggins paper for the 2.2 number.

Presumably more could be found with a very careful search through the papers of Pauling and Huggins, but I've never heard any clear explanations and the documents online don't give them either.