Even--even nuclei in the $A\sim100$ mass region are investigated within the framework of the interacting boson model-1 ({IBM-1}). The study includes energy spectra and electric quadrupole transition properties of zirconium, molybdenum, ruthenium and palladium isotopes with neutron number $N\geq52$. A global parametrization of the {IBM-1} Hamiltonian is found leading to a description of about 300 collective levels in 30 nuclei with a root-mean-square deviation from the observed level energies of 120 keV. The importance of the $d_{5/2}$ subshell closure at neutron number N=56 is pointed out. The geometric character of the nuclei can be visualized by plotting the potential energy surface $V(��,��)$ obtained from the {IBM-1} Hamiltonian in the classical limit. The parametrization established on the basis of known elements is used to predict properties of the unknown, neutron-rich isotopes $^{106}$Zr, $^{112}$Mo, $^{116}$Ru and $^{122}$Pd.

15 pages, 7 figures, 4 tables, accepted for publication in Journal of Physics G