Rubidium has two isotopes, ⁸⁵Rb and ⁸⁷Rb.

$Z$ counts protons, $A$ protons plus neutrons, and $N = A - Z$. Isotopes share $Z$, isobars share $A$, isotones share $N$. Average atomic mass $=$ the sum of (mass $\times$ share). In a mass spectrometer, $r = \dfrac{1}{B}\sqrt{\dfrac{2mV}{q}}$, so $r \propto \sqrt{m}$.