Write the number of groups and period in the modern periodic table. Mention the criteria of placing elements in the
(i) same group and
(ii) the same period. Illustrate your answer with an example for each case.
There are 18 groups and 7 periods in the Modern Periodic Table.
(i) In same group, elements with same valencies are placed.
(ii) In period, elements with same shells are placed.
For example: Let us consider the example of Na (sodium), whose atomic number is 11. Now, Na (11) : 2, 8, 1.
It has one valency so, it is in group I. Also, it has 3 shells. So, its period number is 3.