How did mendeleev organize the elements in the periodic table? how does it differ from the way in which the periodic table is arranged today?

Determine the wavelength of the light absorbed when an electron in a hydrogen atom makes a transition from an orbital in the n=3 level to an orbital in the n=7 level.

When you mate two plants together the terms is called? answer it fast as possible plz! i have a test tomorrow!

In the millikan oil drop experiment they determined that every drop had a charge which was a while number multiple of -1.60x10^-19. if a drop has a total charge of -9.60x10^-19 then how many excess electrons are contained within the drop?