How would I randomize the composition of an atmosphere in my game?

I am making a space game with planets that are randomly generated and I need their atmospheres to be made up of random amounts of different elements. So the thing is that I want the elements to be percentages of the complete atmosphere, an example: oxygen = 30, nitrogen = 20, carbondioxide = 50, hydrogen = 0. All these values should be completely randomized and the sum of them all has to be 100.

I basically want to fill a container to the top with random amounts of set elements, but I don't know how to randomize all of the variables and end up with a fixed sum.

This is my first time submitting anything to StackOverflow so please let me know if there is anything I need to clarify, I've been stuck on this issue for so long without finding any answers so I would appreciate any help, thanks :)

(I am using c# in unity in case that makes a difference)