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

What is orbital? Draw the shapes of 2p and 3d orbitals.

Explain why there is no existence of 2d orbital.

On hybridyzation of one S and one P orbitals we get -

Which of the 3d series of the transition metals exhibits the largest number of oxidation states and why?

Though copper, silver and gold have completely filled set of d-orbitals yet they are considered as transition metals. Why?

Draw figure to show splitting of d-orbitals in a octahedral crystal field.