Why does in the building of an atom, the filling of 4s orbital takes place before filling in 3d orbital ?

Why in the building of the atom, the filing of 4s orbitals takes place before 3d orbital?

Why in building of the atoms the filling of 4s orbital takes place before 3d orbital ?

Why is 4s orbital filled before the filling of 3d orbital?

Account for the following: a. The expected electronic configuration of copper is '[Ar] 3d^9 4s^2' but actually it is '[Ar] 3 d^(10) 4 s^1' b. In building up of atoms, the filling of 4s orbitals occurs before '3 d' - orbitals.

Assertion : Ionisation of transition metals involves loss of ns electron before (n - 1)d electrons. Reason : Filling of ns-orbitals takes placed before (n - 1)d orbitals.

After filling of np orbital, the next orbital filled will be

After filling up ‘4s’ orbital the electron enters: