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The distribution of electrons and holes in the CuO$_2$ plane of thehigh-temperature superconducting cuprates is determined with nuclear magneticresonance through the quadrupole splittings of $^{17}$O and $^{63}$Cu. Based onnew data for single crystals of electron-doped Pr$_{2-x}$Ce$_x$CuO$_4$(x=0,0.05, 0.10, 0.15) as well as Nd$_{2-x}$Ce$_x$CuO$_4$ (x=0, 0.13) the changes inhole contents $n_d$ of Cu 3d$(x^2-y^2)$ and $n_p$ of O 2$p_\sigma$ orbitals aredetermined and they account for the stoichiometrically doped charges, similarto hole-doped \lsco. It emerges that while $n_d+2n_p=1$ in all parent materialsas expected, $n_d$ and $n_p$ vary substantially between different groups ofmaterials. Doping holes increases predominantly $n_p$, but also $n_d$. To thecontrary, doping electrons predominantly decreases $n_d$ and only slightly$n_p$. However, $n_p$ for the electron doped systems is higher than that inhole doped La$_{1.85}$Sr$_{0.15}$CuO$_4$. Cuprates with the highest maximum$T_{\rm c}$s appear to have a comparably low $n_d$ while, at the same time,$n_p$ is very high. The rather high oxygen hole content of the Pr$_2$CuO$_4$and Nd$_2$CuO$_4$ with the low $n_d$ seems to make them ideal candidates forhole doping to obtain the highest $T_{\rm c}$.

Distribution of electrons and holes in cuprate superconductors as determined from $^{17}$O and $^{63}$Cu nuclear magnetic resonance