On what basis do we fill the shells of elements with atomic no. more than 20? (Shouldn’t the electronic configuration of Mn be 2,8,8,7? Why is it 2,8,13,2?)

because electronic confriguration of mn is 1s2 2s2 2p6  3s2 3p6  4s2  3d5 we cannot fill more than 2 electron in 1 orbital s subshell have 1 orbital , p subshell have 3 , d subshell have 5orbital and f have 7 and according to pauli exclusion principal 1 orbital can take max. 2 electrons . we can not fill 13 electron in 3^rd shell (3s2 , 3p2 ).