added electron enters a s orbital. Successive filling of s and p orbitals gives rise to the third period of elements from sodium to argon. The fourth period ( n = ) starts at potassium, and the added electrons fill up the s orbital. Now you may note that before the p orbital is filled, filling up of d orbitals becomes energetically favourable and we come across the so called d transition series of elements.
This starts from scandium ( Z = ) which has the electronic configuration d s . The d orbitals are filled at zinc ( Z = ) with electronic configuration d s . The fourth period ends at krypton with the filling up of the p orbitals. Altogether we have elements in this fourth period.
The fifth period ( n = ) beginning with rubidium is similar to the fourth period and contains the d transition series starting at yttrium ( Z = ). This period ends at xenon with the filling up of the p orbitals. The sixth period ( n = ) contains elements and successive electrons enter s , f , d and p orbitals, in the order — filling up of the f orbitals begins with cerium ( Z = ) and ends at lutetium ( Z = ) to give the f - inner transition series which is called the lanthanoid series . The seventh period ( n = ) is similar to the sixth period with the successive filling up of the s , f , d and p orbitals and includes most of the man-made radioactive elements.
This period will end at the element with atomic number which would belong to the noble gas family. Filling up of the f orbitals after actinium ( Z = ) gives the f - inner transition series known as the actinoid series . The f