Simplify the following text for a Grade 9 native English-speaking person.
Original: Rubidium has a number of oxides , including rubidium monoxide ( Rb2O ) , Rb6O and Rb9O2 , which form if rubidium metal is exposed to air ; rubidium in excess oxygen gives the superoxide RbO2 . Rubidium forms salts with halides , making rubidium fluoride , rubidium chloride , rubidium bromide and rubidium iodide .
Simplified: Rubidium is an alkali metal .