Why are two hydrogen atoms able to combine chemically?

because helium is a 'noble gas' and that it has two electrons. I seriously dont know about why it doesnt react, but i think it's because ov it's protons. All elements are happy with 8 electrons on their outer shell, with the exception ov helium, so they completely repel each other while hydrogen has one to giv away, so they r reactive. The sun also transferrs large amounts of hydrogen into helium by smashing them together. REMEMBER HELIUM IS A NOBLE GAS, so its happy, and completely ineret and aloof. Hope this helps! :) plus hydrogen also haz one electron pon itz outer shell, so the two share.