This really feels like we're going to descend into an "is water wet"-style debate. Those are always fun.
I think I'd still consider it a lick even if you just contact one atom of a substance to your tongue. You could even fire it at your tongue with the LHC and I'd still be willing to go along with calling that a lick, using the term very loosely.
I'm less sure on the gas, but fuck it, sure, sticking your tongue into a gas can be a lick between friends. Alternately you could condense/freeze it and lick that form, but that might be worse.
There's no getting away from the idea that it would be very difficult to lick an atom with sub-millisecond half life. Unless... you create it already inside your tongue! I'm sure we could do that somehow. Probably involving magnets.