The horizontal rows in the long form periodic table are called periods. There are
- The number of main shells present in the atom of particular element decides to which period it belongs to.
Eg : i) Hydrogen (H) and Helium (He) atoms contain only one main shell (K). Therefore they belongs to period - 1. ii)The elements Li, Be, B, C, N, O, F and Ne contain two main shells (K and L) in their atoms. Therefore they belong to period - 2.
- The number of elements in a period depends on how electrons are filled into various shells. Each period starts with a new shell and ends when shell is filled with respect to the ‘s’ and ‘p’ levels of its main shell.
The first period contains two elements because the K shell contains only one sub-shell i.e., Is, which can accommodate 2 electrons only.
- The second period starts with the 2nd main shell (L), which has two sub shells 2s and 2p, which can accommodate 8(2 + 6) electrons. Hence second period contains 8 elements.
- Third period starts with third main shell (M), which has 3 sub-shells 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 by filling 3 s and 3p.
- Fourth main shell (N) has four sub-shells, 4s, 4p, 4d and 4f but while filling electrons, they are filled in the order 4s, 3d and 4p. Due to this, 4th period contains 18 elements (2 + 10 + 6).
- Similarly 5th period contains 18 elements, 6th period contains 32 elements.
- 7th period is incomplete and contains 3 elements from s-block 14 elements from (f-block, 10 elements from d-block, some elements from p-block).
- The 4f elements (Lanthanoids) and 5f elements (Actinoids) are shown separately at the bottom of the periodic table.