The rule for inert gases is 'eight electrons in the outer shell of their atoms' with helium being an exception (because helium has only two electrons in its outer shell). A better rule for inert gases that included helium would be 'zero electrons missing from the outer shell of their atoms'.

'One electron missing from the outer shell of their atoms' also works the same as 'seven electrons present' for halogens except that it allows hydrogen to be classified as a halogen without making an exception (similar to helium for inert gases).

The only problem is the upper limit of eight electrons in the outer shell of any atom. This has to be included with any 'number of missing electrons' rules. It is not a given because electron shells beyond the second may contain more than eight electrons as long as they are not the outermost shell. I did not know about the eight electron limit until I started researching this matter in depth.

Either way hydrogen should be classified as a halogen and positioned on the Periodic Table accordingly, even though hydrogen also fits the rule for the alkali metals (the other elements in Group 1) having a single electron in the outer shell of its atom.