One- and two-body densities in momentum space have been calculated for the atomic beryllium isoelectronic series starting from explicitly correlated multideterminant wave functions. The effects of electronic correlations have been systematically studied by comparing the correlated results with the corresponding Hartree-Fock ones. Some expectation values such as [delta>(*) over bar *((p) over right arrow)], [p(n)], [delta>(*) over bar *((p) over right arrow (12))], [p(12)(n)], [delta>(*) over bar *((P) over right arrow)], and [P-n], where (p) over right arrow, (p) over right arrow (12), and (P) over right arrow stand for the electron-nucleus, interelectronic, and two-electron center-of-mass momentum coordinates, respectively, and the angular correlation coefficient have been obtained. All the calculations have been carried out by using the Monte Carlo algorithm.