Why do some compound have 1 hydrogen, some have 2 hydrogens, and some have 3 hydrogens?

An electron moved from shell n = 2 to shell n = 1. what most likely happened during the transition? a fraction of a photon was added. a photon of energy was absorbed. a fraction of a photon was removed. a photon of energy was released.

  the mississippians were considered to be  horticulturalists, which means they were

Calculate the percent composition by mass of each element in al(oh)3. use at least three significant figures.