Periodic Table
Modern periodic table (Modified Mendeleev Periodic table):
(1) It was proposed by Moseley in 1913.
(2) Modern periodic table is based on atomic number.
(3) Moseley did an experiment in which he bombarded high speed electron on different metal surfaces are obtained X-ray.
He found out that 
where v≈ frequency of X-rays, Z – atomic number from this experiment, Mosley concluded that the physical and chemical properties of the elements are periodic function of their atomic number. It means that when the elements are arranged in the increasing order of their atomic number then elements with similar electronic configuration are repeated after a regular interval of time.This is also known as ‘Modern Periodic Law’.
(4) Modern periodic law: The physical and chemical properties of all the elements are the periodic function of their atomic number.
Long form \ Present form of modern periodic table:
(Also known as ‘Bohr, Bury Periodic Table)
(i) It is based on the electronic configuration concept and atomic number.
(ii) This model is proposed by Rang & Werner.
(iii) 7 periods and 18 vertical columns (groups).
(vi) According to IUPAC 18 vertical columns are named as 1st to 18th
(v) Elements belonging to same group having same number of electrons in the outermost shell so properties are similar.
| Period | N | Period Sub shell | No. of elements | Element | Name of period |
| 1. | 1 | 1s | 2 | 1H, 2He | Shortest |
| 2. | 2 | 2s, 2p | 8 | 3Li – 10Ne | Short |
| 3. | 3 | 3s, 3p | 8 | 11Na – 18Ar | Short |
| 4. | 4 | 4s, 3d, 4p | 18 | 19K – 36Kr | Long |
| 5. | 5 | 5s, 4d, 5p | 18 | 37Rb – 54Xe | Long |
| 6. | 6 | 6s, 4f, 5d, 6p | 32 | 55Cs – 86Rn | Longest |
| 7. | 7 | 7s, 5f, 6d | 26 | 87Fr – 112Uub | Incomplete |
Conclusion:
1. Period number = outermost shell
2. Number of element in a period = Number of electrons in a period subshell
(vi) Identification of group, period and block:
(A) When atomic number is given:
Step I: 71 Z 58⇒ Lanthanoids (6th period) f-block
103 Z 90⇒ Actinoids (7th period)
Step II: Z = 104 to 118 (Period number 7)
Group number = last two digits in atomic number of element.
Step III: Group number = 18 + given atomic number – atomic number of next noble gas
If the value of this formula is negative then use 32 instead of 18 in formula.
(B) When electronic configuration is given:
Period number = number of outermost shell
Block Identification:
– If np electron present then p – block (ns2 np1-6)
group number = 12 + np electrons
– If np electron absent then s/f/d block
If (n-2)f0 (n-1)d0 ns1-2 = s block
group number = ns electrons
If (n-2)f1-14 (n-1)d0-1 ns2 = f block
group number = IIIB
– If any other configuration or (n-1)d1-10 ns0-2 (d-block)
group number = (n-1)d electron + ns electron
Electronic configuration of elements and the periodic table:-
(a) Electronic configuration of elements in periodic:- Each period in the periodic table indicates the value of n = for the outermost or the valence shell. In other words, successive periods in the periodic table, i.e. 1, 2, 3, …. etc. refer to the filling of 1st , 2nd , 3rd and higher principal energy levels.
The first three periods containing 2, 8, 8 elements respectively are known as short periods while the next three periods containing 18, 18, 32 elements respectively are called long periods.
(b) Groupwise Electronic Configuration:- The elements in the same group or vertical columns have similar valence shell electronic configuration, the same number of Electrons in the outer orbital and similar properties.