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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Priyanshu kumar
Dear student, Filling of orbitals takes place by considering aufbau rule(n+l) ...The orbitals whose n+l value is smaller filled first.... Between 4s and 3d ...4s has n+l value=4+0=4 and 3d has n+l value=3+2=5...so 4s orbital is filled according to aufbau rule...... Now come to the situation of ionisation as for greater value of principal quantum number 'n' the outermost orbital is decided as it has maximum distance from nucleus and influence of nucleus or we can say nuclear charge is less effectively pronounced for outermost orbital...So 4s is outermost orbital here..So removal of electrons takes place firstly from 4s orbital then 3d orbital...
Means fillings of orbitals and removal of electrons are working for 4s and 3d in opposite manner...
Hope this will helps your queries...