The quadrupole distribution of the weak nuclear charge causes P-odd mixing of s_1/2 and p_3/2 atomic states. The measurement of the corresponding P-odd effects in atoms would allow to determine the neutron quadrupole moment of a nucleus. The experiments with rare earths are of a particular interest in this respect. A simple derivation is presented for the imitating effect which originates from the combined action of the total weak charge and the quadrupole hyperfine interaction.