  The shell effect and isospin effect in nuclear charge radii are
systematically investigated and a four-parameter formula is proposed for the
description of the root-mean-square (rms) charge radii by combining the shell
corrections and deformations of nuclei obtained from the Weizs\"acker-Skyrme
mass model. The rms deviation with respect to the 885 measured charge radii
falls to 0.022 fm. The proposed formula is also applied for the study of the
charge radii of super-heavy nuclei and nuclear symmetry energy. The linear
relationship between the slope parameter L of the nuclear symmetry energy and
the rms charge radius difference of 30S - 30Si mirror pair is clearly observed.
The estimated slope parameter is about $L=54 \pm 19$ MeV from the coefficient
of the isospin term in the proposed charge radius formula.
