Energies of the 1s22s2S1/2 and the 1s23s2S1/2 of the lithium isotopes are calculated in a dynamic correlation model. With the evaluated configuration mixing wave functions the polarization effects resulting from an extended nuclear charge distribution are evaluated. A relative large contribution for 7Li is observed.PACS Nos.: 32.10.Fn, 21.10.Ft, 27.20.+n