What is a quantum-mechanical orbital?

Solution We have been asked to arrange the set of atoms from largest to smallest. Step1 a)K,Li and Cs belongs to group 1 A of the periodic table.When we come down along the group, size of the atom increases .This is because a new shell is being added which results in an increase in screening effect.Thus the effective nuclear pull experienced by the outermost electron decreases. Li comes at the top of the group followed by K and Cs .Thus the decreasing order of atomic size is Cs>K>Li