Structural Features of the Modern Periodic Table
This table consists of horizontal rows called as 'periods' and vertical columns called as 'groups'.
Periods
There are seven periods in the periodic table and each period starts with a different principal quantum number.
The first period corresponding to 'n' = 1 consists of only two elements hydrogen and helium .This is because the first energy shell can accommodate only two electrons.
In the second period corresponding to 'n' = 2, with a capacity of eight electrons and so contains eight elements. This period starts with lithium (Z = 3) ends with neon (Z = 10) where the second shell is complete.
In the third period corresponding to 'n' = 3, contains eight elements. It starts with sodium (Z = 11) and ends with argon (Z = 18) where the third shell is partially complete.
The fourth period corresponding to 'n' = 4, consists of eighteen elements in this period starting from potassium (Z = 19) to krypton (Z = 36) where the third shell gets completed.
In the fifth period there are 18 elements like the fourth period. It begins with rubidium (Z = 37) and ends with xenon (Z = 54).
The sixth period contains 32 elements (Z=55 to 86) .It starts with caesium and ends with radon.
The seventh period, though expected to have 32 elements is incomplete and contains only 19 elements at present.
The first three periods are called short periods while the other three periods are called long periods.
Groups
The vertical column in the periodic table is called as group. There are 18 groups in the long form of the periodic table and they are numbered from 1 to 18 in the IUPAC system. In the old system of naming they are numbered as I to VIII with A and B groups.
