At ambient conditions, alkali metals adopt the body centered cubic structure, while if compressed up to tens of GPa and above, they exhibit complex low-symmetry modifications, due to the density-driven transition of the valence electrons from the s state to states of higher angular momentum. These high-pressure, low-symmetry phases, whose unit cells may include up to tens of atoms, allow rich Raman activity, which was previously observed only in lighter alkalis Na and Li. Here we report an extensive study of the optical phonons of highly dense Rb up to 100 GPa in diamond anvil cells, conducted by challenging experimental Raman spectroscopy measurements and ab initio computer simulations. The relative (relative to the normal condition value) density behavior of Raman frequencies of Rb is compared to that of Na and Li, once the frequencies of the two light alkali elements have been rescaled by root M-Na/M-Rb and root M-Li/M-Rb, respectively, where M-Na, M-Li, and M-Rb are the atomic masses of the here considered alkali elements. Importantly, while the rescaled density behaviors of Na and Li agree with each other, Rb significantly differs, which highlights the different nature of the valence electron transition being of the s-d and of the s-p type in heavy and light alkali metals, respectively, a result that calls for further similar investigations of K and Cs.