and ends when the main shell is filled with respect to the ‘s’ and ‘p’ subshells (except the first period).

The first period starts with K-shell. The first main shell (K) contains only one sub-shell, the (1s). For this subshell only two types of electronic configurations are possible and they are 1s¹ (H) and 1s² (He). Therefore, the first period contains only two elements.

3. Second period starts with the 2^nd main shell (L). L-shell has two subshells, namely, 2s and 2p. Eight types of configurations are possible in this shell (L) like 2s¹ and 2s² and 2p¹ to 2p⁶ . Hence the second period contains 8 elements Li, Be, B, C, N O, F and Ne in the order given. Thus, the 2^nd period consists two s-block elements (Li, Be) and six p-block elements (B to Ne)
4. Third period starts with third main shell (M). This shell (M) has 3 subshells, namely, 3s, 3p and 3d, but while electrons are being filled into the shell ‘3d’ gets electrons only after ‘4s’ is filled. Therefore, the 3rd period contains again 8 elements, which includes two s-block elements (Na, Mg) and six p-block elements (Al to Ar


5. Fourth main shell (N). This shell (N) has four sub-shells namely 4s, 4p, 4d and 4f, but while electrons are being filled into the shell, electrons enter the atoms in the order 4s, 3d and 4p. Due to this, the fourth period contains 18 elements which includes two s-block (K, Ca), 10 elements from d block (Sc to Zn) and six elements from pblock (₃₁Ga to ₃₆ Kr). There are altogether eighteen elements in the fourth period

On the same lines, we can explain why there are 18 elements in the fifth period (₃₇Rb to ₅₄Xe).

There are thirty two elements in the Sixth period from ₅₅Cs to ₈₆ Rn which includes 2 elements from s-block (6s) 14 elements from f-block (4f) 10 elements from d-block (5d) and 6 elements from p-block (6p).

‘4f’ elements are called Lanthanoids or lanthanides. Elements from