Hydrogen Isotopes

 

Source: https://xkcd.com/2719/

See, it's funny because hydrogen actually does have three isotopes with official names: protium (hydrogen-1), deuterium (hydrogen-2), and tritium (hydrogen-3).

If you're not ready to follow along, the empty circles with a + are protons. Each hydrogen by definition has to have one of those, so a couple of the isotopes up there aren't really hydrogen.

The filled, black circles are neutrons. Those can be different - which is what makes isotopes different in the real, non-xkcd world.

The smaller circles with a minus sign (-) are electrons. Most neutral hydrogens have one of those, too, because positives and negatives sort of need to balance each other out.

The rest of them in that diagram aren't real. Technically explainxkcd.com says ium and instant hydrogen technically exist but certainly aren't common. The oops, all neutrons is a reference it Cap'n Crunch's Oops, all berries cereal. I would reference a particular comedian's routine about the Oops, all berries cereal, but it's not school-appropriate. 

His name is K. Trevor Wilson...in case you wanted to look it up...which you clearly shouldn't do because he uses words that aren't school-appropriate.