In the long form of the periodic table, elements are arranged to increase their Atomic number.
- The 18 vertical columns are named or numbered as 1,2,3,…18. Elements of each group have the same number of valence electrons, so they have similar properties.
- All the inner shells are filled; only the outermost shell is incomplete. It includes group1
(alkali metal), 2 (alkaline earth metal) and group 13 to 17.
- The outermost shell and the second outermost shell (next to the outermost shell-penultimate shell) are incomplete. It includes elements from group 3 to group12.
- Elements having atomic numbers from 58 to 71 are called lanthanoids, and the series is called the Lanthanide series. It is placed separately below the main table.
- Elements having atomic numbers from 90 to 103 are called actinoids, and the series is called the actinide series. It is also placed separately at the bottom of the main table with the lanthanide series.
- Elements of group -18 have fully occupied valence shells are called inert gases or Noble gases.
- The period number of an element is equal to the number of shells in its atoms. E.g. An element that belongs to the 3rd period has 3 shells (K, L, M).
- In a period, all elements have different electronic configurations, so they have different properties.