(a) assuming an atomic nucleus is spherical in shape, show that its radius r is proportional to the cube root of the mass number (a). (b) in general, the radius of a nucleus is given by r roa1/3, where ro is a proportionality constant given by 1.2 x 10^-15 m. calculate the volume of the li nucleus. (c) given that the radius of 7li3 atom is 152 pm, calculate what fraction of the atom's volume is occupied by its nucleus. does your result support rutherford's model of the atom?