why are modern airships filled with helium?
Hydrogen is lighter than helium, helium is the preferred lifting agent. Helium is not flammable like hydrogen and it is preferred for safety reasons. Although Hydrogen has a higher lifting power the difference is inconsequential because they are both still lighter than air agents.
why wasn't the Hindenburg filled with helium instead of hydrogen?
At the time the Hindenburg was being prepared for flight the United States controlled most of the helium market. This posed a problem because the United States had an embargo on Germany, making it nearly impossible to obtain the helium needed. The company in charge of the Hindenburg had no choice other than to proceed with Hydrogen.
Why is helium safer than hydrogen for use in airships? (using orbital diagrams)
Both Hydrogen and Helium are lighter than air, they are both colorless and non toxic. What makes Hydrogen particularly combustible is its small molecular size, diffusivity and bouncy. 0.002mL of energy are required to ignite hydrogen, one of the other factors is its 74% flammability range.
On the Left: Hydrogen Orbital Diagram On the Right: Helium Orbital Diagram
The relationship shown in the diagrams shows that helium is safer than hydrogen to use for airships because the orbital diagram of helium shows that it has more control because it can pressurized to go up and down. Hydrogen, however, can only be used moving an airship up. The more control a pilot has, the safer it is for the passengers.
Sources:
✿ http://us.123rf.com/400wm/400/400/designua/designua1209/designua120900002/15013058-structure-atom-helium-and-hydrogen.jpg
✿ http://preview.turbosquid.com/Preview/2010/12/21__09_26_24/balloon2b_render.jpg309351fb-40d0-4314-8fa3-1ef5ff3ba4d4Large.jpg
✿ http://teachers2.wcs.edu/high/chs/kimm1/Honors%20Portfolio/4th%20Nine%20Weeks%20Portfolio%20Documents/ChemMatters%20Articles/Mathematics%20of%20Chemical%20Equations/Hydrogen%20and%20Helium.pdf
✿ http://www.arhab.org/pdfs/h2_safety_fsheet.pdf