A horizontal row of elements in the periodic table is called a period. There are seven periods in the periodic table. Each period starts with Group 1(alkali metal group), and ends up with Group 18(noble elements group).
1. The first period elements (H and He) have only one shell- K-shell. K-shell starts getting filled at H (K ;1), and is completely filled atomic He(K;2).
2. The second period elements (Li, Be, B, C, N, O, F and Ne) have two shells- K and L shells. The L-shell starts getting filled with Li(K,L:2,1), and is completely filled with Ne(K,L:2,8).
3. Third period elements have three shells – K, L and M shells. It starts with Na(K,L,M;2,8,1), and ends up with Ar(K,L,M;2,8,8).
Similarly, the fourth period elements have four shells- K, L, M and N. and so only.