Because a property should equal given value: a%26b%3Dc%3F (or a&b=c? decoded). You shouldn't be decoding the string until you're assigning its value, that's why encoding is there to begin with, so there's no conflicts with charaters = or & when used as values. (I know you solved it already, this is for anyone else who gets confused)

nope, Uun is a perfectly valid chemical element: it's one of those that are at the extreme end of the table (often not even written in it)