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.
By that logic you could argue that you're always licking carbon since your tongue has a lot of carbon in it. But I think the carbon has to be pure for it to count. If it's in a compound molecule it's void