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Tuesday, July 27, 2021

Effective Nuclear charge (Z* or Zeff): Slater's rule: Screening effect or Shielding effect

The decrease in the force of attraction exerted by the nucleus on the valence electrons due to the presence of electrons in the inner orbit is called “Screening effect or shielding effect”


Due to shielding effect of inner shell electrons on the outer shell electrons such that valence electrons feel less attraction from the nucleus. This decreased nuclear charge is known as effective nuclear charge (Z* or Zeff)

The Slater’s  rule determine the actual charge felt by an electron and also allow you to estimate the effective nuclear charge Zeff  from the real number of protons in the nucleus and the effective shielding of electrons in each orbital "shell"

“Slater defined Z* or Zeff (effective nuclear charge) as a measure of nuclear attraction for an electron. Or The magnitude of positive charge “experienced” by an electron in the atom.

                                              Thus Z* =Z-S

Where Z is the (nuclear charge)= The number of protons in the nucleus of an atom,  and S represent the shielding constant and value is greater than zero but less than Z.

Step 1: Write the electron configuration of the atom in the following form:

              (1s) (2s, 2p) (3s, 3p) (3d) (4s, 4p) (4d) (4f) (5s, 5p) . . .

Step (2):  Electron in higher group do not shield the electrons in the lower group.

Step (3): For ns and np valence electrons:

(A) The electrons in same ns and np group contribute 0.35, except the 1s which shield 0.30

(B) Electrons within the n-1 group contribute 0.85

(C) Electrons within the n-2 or lower groups contribute (shield) 1.00

Example(1) :  Calculate the Zeff (Z*) for 4s electron of chromium atom (Slater's Rule)?

Step (4): For nd and nf valence electrons:

(A) Electrons in same nd and nf group contribute 0.35

(B) Electrons in the group to the left contribute 1.00

Shielding constant (S) obtained is subtracted from Z to get Z*

Example(2) :What is the shielding constant (S) experienced by a 3d electron in the bromine atom?


Periodicity in Z* or Zeff:

(A)  In a period: When we move from left to right in period the Z* increases.

2nd (P)

Li

Be

B

C

N

O

F

Z*/Zeff

1.30

1.95

2.60

3.25

3.90

4.25

5.20

 

(B)  In a Group: When we move from top to bottom the Z* remains constant,

1st gp

Li

Na

K

Rb

Cs

Z*/Zeff

1.30

2.20

2.20

2.20

2.20


Related Questions:

(1) What are the Amphoteric metals ? gives Examples.

(2) Name of total metalloids present in periodic table ?

(3) Total numbers of elements which are liquid at normal temperature is ?

(4) What is Mendeleev's periodic table ? give important features and draw back of Mendeleev's table.

(5) What is atomic density ? give the periodicity of atomic density in periods and groups.

(6) What is atomic volume ? and what is periodicity of atomic volume in groups and periods ?

(7) Why there are 2, 8 and 8 elements in first, second and third periodic of periods table respectively ? Explain.

(8) In alkali metal group which is the strongest reducing agent in aqueous solution and why?

(9) The electron affinity of sulphur is greater than oxygen. Why?

(10) The first ionization energy of carbon atom is greater than that of boron atom, whereas reverse is true for the second ionization energy. Explain.

(11) The electronegativities of B, Al, Ga are 2.0, 1.5, 1.6 respectively. The trend is not regular. Explain.

(12) Li2CO3 decomposes on heating but other alkali metal carbonates don’t. Explain.

(13) Of all noble metals, gold has got a relatively high electron affinity. Explain.

(14 Ionization energy of Boron is smaller than Beryllium even though effective nuclear charge is higher?

(15) What are the increasing order of ioni radii of first group elements in water ?

(16) What are the increasing order of molar conductivity of first group elements in water ?

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