Two hydrogens were walking down the street then one hydrogen looked at the other and said I think I lost an electron and so the other hydrogen said Are you sure? and the other hydrogen said Im positive.
The first time I heard this joke from Raksha was when she told it during a study break one night when both of us and some of our friends were in a study lounge. I was doing some homework while Raksha and some other people were studying for a chemistry exam. After studying for awhile, Rakshas chemistry study group decided to take a break. At one point, the conversation became about the chemistry review sessions, and then Raksha began repeating the chemistry jokes that her review leader had told them. These jokes relaxed the students and provided them with a mental break, taking their minds off of their upcoming chemistry exam.
For those that are unfamiliar with chemistry, a hydrogen atom has an electron that carries a negative charge, a proton that carries a positive charge, and a neutron, which is neutral. If hydrogen loses an electron, then it will only have a proton and a neutron; therefore, it only has a positive charge. That is why the punch line of the joke is Im positive, having a double meaning that the hydrogen is certain he is without an electron and since he is without an electron, he is positively charge.
Most likely, this joke would require an explanation for its humor to the people who do not understand hydrogen atoms and chemistry. Since only students who have studied chemistry would understand this joke, this joke serves as a function like occupation or ethnic folklore. The chemistry students are like a group and only its members will understand why this joke is funny. This joke could help identify who are science students and who are not. If one laughs and finds this joke entertaining, then he/she must have some knowledge of chemistry, whereas, if one does not comprehend the joke, he/she most likely does not know chemistry.