Simplify the following text for a Grade 7 native English-speaking person.
Original: Polonium is a chemical element with the symbol Po and atomic number 84 , discovered in 1898 by Marie SkÅ odowska-Curie and Pierre Curie .
Simplified: It is chemical element 84 on the periodic table and its symbol is Po . It was discovered in 1898 by Marie and Pierre Curie .