Discuss the relation between the number of electron filled into the sub energy levels of an orbit and the maximum number of elements present in a period.

Explanation

Construction of periods:
(a) 1st period (short period) consists of two elements `H_(1)` and `He_(2)`. The K shell (n=1) can have a maximum of two electrons only. So there are only two elements in this period.
(b) 2nd period (short period) contains 8 elements namely `Li_(3)` to `Ne_(10)`.
Li atom has completed K-shell and a new shell, L starts with one electron in the remaining elements of the period i.e., from Be to Ne, the L shell is gradually filled up till Ne is reached. In Ne, the K as well as L shells are both completed.
Thus, the period ends. In these elements, the second energy level (L) gets gradually filled with a maximum of 8 electrons. So this period contains 8 elements.
(c ) 3rd perood (short period) also contains 8 elements , i.e., `Na_(11)` to `Ar_(18)`.
The M shell starts filling up with sodium. The shell builds up steadily, until argon is reached. After argon, the differentiating electron does not enter M shell (i.e., 3rd shell) but enters a new shell i.e., N shell (4th shell). Hence the 3rd peirod has only 8 elements.
(d) 4th period (long period) has 18 elements which are `K_(19)` and `Kr_(36)`
Then N shell starts filling in K (potassium). K hs configuration 2,8,8,1. In Ca. anthor electron enters the N shell. It has the configuration 2,8,8,2.
Starting with the next element Sc(Z = 21). The penultimate M shell is expanded till it is complete with 18 electrons. In Cr and Cu . only one electron is present in the outermost shell i.e., `N^(th)` shell. While all the other elements contain 2 electrons. With Zn, the M shell is complete.
Then the extra electron enters the outermost `(N^(th))` shell in the successive elements Ga to Kr. In this 4th period. 4s, 3d, 4p levels are successively filled with electrons. Hence this period has 18 elements.
(e ) 5th period follows closely the sequence in 4th period. It starts filling with 5s level. In ends when the 5p level is complete. The sublevel 5s, 4d and 5p get filled up. Then the total number of electrons filled into these levels is 18 and hence there are 18 elements in this period. The period starts with Rb and ends with Xe.
(f) 6th period is long. In it, 6s, 4f 5d and 6p energy levels get filled, which includes the 14 lanthanides. The total number of electrons filled in these levels is 32. Hence the total number of elements in the period is 32.
(g) 7th period is incomplete, which includes the 14 actinides. There are nearly 20 elements in this period.