What information could you use to determine the atomic number of an unknown element that has a mass number of 22?

The atomic number is equal to the amount of protons in an atom. The mass number is the amount of protons and neutrons in an atom. 
If you know that the mass number is 22, you would need to know the amount of neutrons to be able to solve for the amount of protons (AKA atomic number).

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.