what is periodicity

introduction:

Periodic table is design in such a way so that we can understand all the physical and chemical properties in a proper order . Elements having similar properties are placed either in same group or same period as well as.

Periodicity :

The repetition of physical and chemical properties after the regular time interval is defined as periodicity of periodic table. Basically it depends on many factor i.e screening effect,electron affinity and electronegativity etc.

Screening effect or Shielding effect

The phenomena of decrement in the force of attraction between the outer electron and nucleus because the nucleus and the valence shell electron due to inner electron is defined as screening effect or shielding effect.

effective nuclear charge

It is defined by slater
                                   

                                   Z* = Z - σ
here Z* is effective nucleus charge , Z is nuclear charge and σ is screening const or slater const .

The reduce force of attraction by the nucleus on the valence electron (see in above figure) is defined as effective nuclear force. It is discuss in term of charge so it is named as effective nuclear charge.

Calculation of screening effect of any element through following  slater rule:

• Arrange the orbital in the given form

             (1s) (2s 2p) (3s 3p) (3d) (4s 4p) (4d 4f)

• The amount of extent from which the force of attraction decreasing is named as screening constant (σ ) .
• The value of screening constant for the different shell are : -

         

Shell	(n-4)th	(n-3)th	(n-2)th	(n-1)th	nth
σ	1	1	1	0.85	0.35

Let's take some example to more understanding

Ex.     1.  Li ➝ 1s² 2s¹
                      σ = 2 x 0.85 = 1.7     so,  Z*= Z- σ = 3-1.7=1.3
  here we are finding screening const of last electron so we have not consider n^th  electron.
 Same things have done in upcoming example .

          ^{2. Na ➝}  1s²   2s² 2p6  3s¹
                           ↑          ↑        ↑
                       (n-2)     (n-1)    n

                  σ = 8 x 0.85 + 2 x 1 = 8.8

                so  Z*= 11-8.8 =  2.2

           ^{3. K ➝}  1s² 2s² 2p6     3s² 3p6     4s¹
                             ↑                  ↑
                         inner              (n-1)

                  σ = 8 x 0.85 + 2 x 1 + 1 x 0.35 = 9.15

                  so  Z*= 12-9.15 =  2.85


           ^{4. Mg ➝}  1s²   2s² 2p6  3s²
                             ↑          ↑        ↑
                         (n-2)     (n-1)    n

                  σ = 8 x 0.85 + 2 x 1 + 1 x 0.35 = 9.15

                  so  Z*= 12-9.15 =  2.85

           ^{5. Ca ➝}  1s²    2s² 2p6   3s² 3p6   4s²
                           ↑           ↑            ↑           ↑
                         (n-3)     (n-2)      (n-1)       n

                  σ = 8 x 0.85 + 10 x 1 + 2 x 0.35 = 17.15

                so  Z*= 20-17.15 =  2.85

let's try it

Find the screening constant and then effective nuclear charge for the last for the last e- of :
(a)  Zn       (b)  B     (c)   N

Check solution  for more clear :  

https://drive.google.com/open?id=1DX-f57Vg1l7FZStN7xwYK2SEWpKpwnYX

As you can see in above example of  Na , K and Ca ( which belongs to same group i.e 1st group ).
The Z* (Effective nuclear charge ) of all the elements are almost same .So we conclude a important note that there is no change in Z* in group .

Now take some example in the same period elements.

1. B = 1s²   2s² 2p¹
           ↑         ↑       
          n-1       n
 σ = 2 x 0.85 +  2 x 0.35 = 2.40
Z* = 5 - 2.40 = 2.6

2. C = 1s²   2s² 2p²
           ↑         ↑       
          n-1       n
 σ = 2 x 0.85 +  3 x 0.35 = 2.75
Z* = 6 - 2.75 = 3.25

3. N = 1s²   2s² 2p³
           ↑         ↑       
          n-1       n
 σ = 2 x 0.85 +  4 x 0.35 = 3.1
Z* = 7- 4 = 3.9

B   --    C    --    N    --    O           here consecutive difference = 0.65
↑           ↑           ↑            ↑
2.60    3.25     3.90        4.55   

so we get another important point

Note : In period ,Z* increases by 0.65 for each movement in period.

So we have seen how to find screening constant and effective nuclear charge for a particular elements . These concept are very important to understanding other periodic properties in the table.

Some of the important properties like lanthanide construction , we will see in upcoming post.