  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
$V(\beta,\gamma)$ 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.
