Modern periodic table question -C
Screening effect or shielding effect can be defined as a reduction in the effective nuclear charge on the electron cloud. This is caused by the difference in the attraction forces of the electrons on the nucleus.
The electron present in the valence shell (ultimate shell) experience lesser nuclear charge due to the electrons present in the inner shells thus providing a "shielding" against outer electron from being influenced by nuclear charge .
Cs = 1s22s22p63s23p64s23d104p65s24d105p66s1
K = 1s22s22p63s13p64s1
As you can see there are more orbitals separating the valence electrons from the nucleus of the atom, so the nucleus will have less control of the 6s1 electron than it will of the 4s1 electron as the shielding of 4d orbitals will cause screening.. This is the shielding affect.Regards