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1. The modern periodic table is divided into two main categories:
(i) Vertical columns called Groups and
(ii) Horizontal columns called Periods.
2. There are 18 vertical columns or groups. These are further sub-divided into A and B (groups I to VII), VIII group and zero group.
3. Member of the same group have similar electronic configuration of the valence shell and thus show same valency.
4. Elements of groups IA to VIIA are called groups of typical elements, representative elements or normal elements.
5. Groups IA and IIA are strongly metallic and are called group of 'alkali metals and alkaline earth metals'.
6. Groups IB to VII B and VIII lie in the middle of the table between IIA and IIIA groups and are called groups of transition elements. They consist of metals.
7. The Zero group consists of 'Noble gases'.
8. There are 7 horizontal rows in the periodic table. These are called the periods.
9. In a period, the number of valence shell remains the same for all elements. However, the number of electrons in the valence shell increases from left to right .
10. The number of elements present in each period is given in the following table.
| Period | Valence shell | Type of Period | Number of elements | Atomic Number of elements |
|---|---|---|---|---|
| 1st Period | n = 1 | Short period | 2 | Atomic number 1 and 2 |
| 2nd Period | n = 2 | Short period | 8 | Atomic number 3 to 10 |
| 3rd Period | n = 3 | Long period | 8 | Atomic number 11 to 18 |
| 4th Period | n= 4 | Long period | 18 | Atomic number 19 to 36 |
| 5th Period | n = 5 | Long period | 18 | Atomic number 37 to 54 |
| *6th Period | n = 6 | Long period | 32 | Atomic number 55 to 86 |
| *7th Period | n = 7 | Incomplete | 23 | Atomic number 87 to 109 |
11. The 6 th period consists of elements that have atomic numbers 58 to 71. They are called Lanthanides. The 7th period consists of elements that have atomic numbers 90 to 105. They are called Actinides. Both of them are called inner transition elements.
12. The 7 th period is an incomplete period as it has only 23 elements.
13. Lanthanides and actinides are not accommodated in the main body of the periodic table but are placed separately at the bottom of the table.
14. The position of hydrogen is not certain. Thus it can be placed in both group IA and group VIIA.
15. Group VIIA elements are called halogens or salt producers. Representative periodic table for eight groups up to calcium (atomic number 20) with their electronic configuration is given in the table.
Representative periodic table for elements with atomic number from 1 to 20 with their electronic configuration
| Group --> Period | 1A | 2A | 3A | 4A | 5A | 6A | 7A | 0 |
|---|---|---|---|---|---|---|---|---|
| Period 1 | 1H 1 | 2He 2 | ||||||
| Period 2 | 3Li 2,1 | 4Be 2,2 | 5B 2,3 | 6C 2,4 | 7N 2,5 | 8O 2,6 | 9F 2,7 | 10Ne 2,8 |
| Period 3 | 11Na 2,8,1 | 12Mg 2,8,2 | 13Al 2,8,3 | 14Si 2,8,4 | 15P 2,8,5 | 16S 2,8,6 | 17Cl 2,8,7 | 18Ar 2,8,8 |
| Period 4 | 19K 2,8,8,1 | 20Ca 2,8,8,2 |
