There are seven periods in periodic table i.e 1st, 2nd, 3rd, 4th, 5th, 6th and 7th.
Ist Period: This is known as shortest period and consists of only two elements i.e Hydrogen and Helium.
2nd And 3rd periods: They are known as short periods, because they consist of eight elements i.e
Li, Be, B, C, N, O, F, Ne (2nd period)
Na, Mg, Al, si, P, S, Cl, Ar (3rd period)
4th And 5th periods: They are known as long periods because each period consists of 18 elements as under.
4th period starts from k to kr and 5th period starts from Pb to Xe.
6th Period: It is known as longest period because it consists of 32 elements from Cs to Rn.
7th period: It is known as incomplete period. It starts from Sr to ........
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