The lanthanide elements react with oxygen to give, generally, compounds of the type Ln₂O₃ (where Ln stands for a lanthanide element). However, there are interesting exceptions, such as a common oxide of terbium, Tb_xO_y. Given that the compound is 73.945% Tb, what is its formula? What is the oxidation number of terbium in this compound? Write a balanced equation for the reaction of terbium and oxygen to give this oxide.