In second shell, electrons start to fill from Li and it gets 8 electrons up to Ne (atomic number 10). So second period contains 8 elements Li, Be, B, C, N, O, F and Ne.
In third shell, electrons start to fill from Na and get 8 electrons up to Ar (atomic number 18) and contain 8 elements Na, Mg, Al, Si, P, S, CI and Ar.
Similarly, 4th, 5th, 6th and 7th periods are completed with different number of electrons. In this way number of elements in different periods is as follows:
Description of Groups:
Vertical columns are called as groups. There are 18 groups in all.
1. 1st group contains seven elements Hydrogen, Lithium, Sodium, Potassium, Rubidium, Caesium and Francium, i.e., H, Li, Na, K, Rb, Cs and Fr.
These elements are called as alkali metals and this group is also called as alkali metal group, because the elements of 1st group easily form alkalies (strong bases) on reaction with water.
2. 2nd group contains 6 elements Beryllium, Magnesium, Calcium, Strontium, Barium and Radium, i.e., Be, Mg, Ca, Sr, Ba and Ra.
These elements are called as “Alkaline-earth metals” and this group is called “Alkaline-earth metal group”, because their oxides are alkaline in nature and present in earth.
3. 13th to 17th, all the groups contain 5-5 elements each. 17th group contains 5 elements Fluorine, Chlorine, Bromine, Iodine, Astatine or F, CI, Br, I and At.
These elements are called as halogens and this group is called as ‘halogen group’ because the elements react with metals to form salts (halo = salt, gene = producer).
4. 18th group or 0 (zero) group contains 6 elements, Helium, Neon, Argon, Krypton, Xenon and Radan or He, Ne, Ar, Kr, Xe and Rn. The elements are gases and do not react with any other element in normal conditions, so they are called as Inert gases or noble gases.
5. Between 2nd and 13th groups, there are 10 elements each in periods 4 to 6. These elements are called Transitional elements.
6. In group 3rd and 6th period, there are 14 elements. They are called Lanthanides. In the same group with 7th period also there are 14 elements. They are called as Actinides.