Question
Account for the fact that the 4th period has eighteen and not eight elements.
Solution
In the long form of the periodic table, each period starts with the filling up of a new principal shell. Clearly, the 4th period will begin with filling of 4th shell i.e. N shell. Thus, fourth period starts with the filling up of 4s orbtial (n = 4). After filling 4s orbital, the filling of 3rd and then 4p takes place. It is so because enthalpy of 3d sub level in between 4s and 4p sub levels. As,
4s = 2 electrons
3d = 10 electrons 
4p = 6 electrons
_______________
18 electrons
Therefore, 18 elements (not eight) are present in the 4th period. The 4d and 4f sublevels are higher in enthalpy than 5s and hence are filled up in the next periods.
