Out of 3d and 4s orbitals, which is filled first ?

Out of 4s and 3d, which subshell is filled first and why?

Among 3d, 4s, 4p orbitals, which one fills last by an electron? And why?

Anitha argued that the 3d orbital is filled first, but not 4s orbital, with electrons. By asking some questions Saritha corrected Anitha's argument. What would be those questions ?

Among 3d, 4s, 4p orbitals, which one has least orbital energy ? Why ?

Out of 3d and 4s, which has more (n + l) value ? Explain.

While filling up electron in the atomic orbitals, 4s orbital is filled before 3d orbital but reverse happens during the ionisation of the atom. Explain why ?

In 4s, 4p, 4d, 3d orbitals, which one has least energy ?

Among 3p, 4s, 3d, 4p orbitals which has least energy ?

A student argued that the 3d orbitals will be filled only after the 4s orbital is completely filled in accordance with aufbau principle. Then another student opposed by saying that it is not true for certain elements like Cr and Cu. Write electronic configuration of underset(24)(Cr) and underset(29)(Cu) . Justify your answer.

A student argued that the 3d orbitals will be filled only after the 4s orbital is completely filled in accordance with aufbau principle. Then another student opposed by saying that it is not true for certain elements like Cr and Cu. Whose argument is correct?