Question

Rubidium has a mass number of 87 and an atomic number of 37. how many electrons does it have?

Answer 1

Rubidium's (Rb) Z = 37
Z ( atomic number ) means , No: of proton or no: of electrons.
.: Number of electrons = 37

Answer 2

Rubidium has 37 elections. there are multiple other sources you could use too confirm this. :-)