s has the greatest effect to reduce the force of attraction between the outermost electron and nucleus due to its effective charge density followed by p>d>f. So, according to the screening effect, s orbital being close to the nucleus lacks screening from any other orbital as it is the first and the closest orbital. p orbital electrons are loosely bound with greater size than s block elements as they undergo screening from s orbital electrons. Similarly, d-orbital electrons undergo screening from s and p-block orbitals and finally f orbital also undergoes screening from all the other orbitals. Hence, the f-block elements are generally big in size with less nuclear charge acting on their outermost electrons. Thus, the order of screening effect of electrons of s,p,d and f orbitals of a given shell of an atom on its outer shell electrons is s>p>d>f.