what is the purpose of a domain in science? do not copy any thing of the internet and they are not the right answer. I am learning classification right now and I need HELP!

Wave functions describe orbitals in a hydrogen atom. each function is characterized by 3 quantum numbers: n, l, and ml. if the value of n = 2: the quantum number l can have values from to . the total number of orbitals possible at the n = 2 energy level is .