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Periodic Table
 Modern Periodic Law and can be stated as : The physical and chemical
properties of the elements are periodic functions of their atomic numbers
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, s-block, p-block, dblock and f-block depending on the type of atomic orbitals that are being
filled with electrons
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Atomic Radius
The atomic radius is defined as one-half the distance between the nuclei of identical atoms
that are bonded together
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Fig
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Variation of Atomic Radii across the period:
The atomic radius of atoms generally decreases from left to right across a period
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Within a period, protons are added to the nucleus as electrons are being
added to the same principal energy level
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Since the force of attraction between nuclei
and electrons increases, the size of the atoms decreases
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Table 3
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As
the atomic number increases down a group, there is again an increase in the positive nuclear
charge
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Higher principal energy levels consist of orbitals which are larger in size than the orbitals
from lower energy levels
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Table: Atomic Radii/pm Down a Family
Atom
(Group I)

Atomic Radius

Atom
(Group 17)

Atomic Radius

Li

152

F

72

Na

186

Cl

99

K

231

Br

114

Rb

244

I

133

Cs

262

At

140

Electron Shielding and Effective Nuclear Charge
If an electron is far away from the nucleus, then there will be electrons present
between the last electron and the nucleus
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As a
result, the electron farther away experiences an effective nuclear charge ( Zeff) that is less than

the actual nuclear charge Z
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 Definition: Shielding The electrons present between the nucleus and last electron,
which lowers the effective charge of the nucleus on the outer electrons
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 Zeff can be calculated by subtracting the magnitude of shielding from the total nuclear
charge and the effective nuclear charge of an atom is given by the equation:

Zeff = Z − S
 where Z is the atomic number (number of protons in nucleus) and
 S is the shielding constant and is approximated by number of electrons between the
nucleus and the electron in question (the number of nonvalence electrons)
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Variation of effective nuclear charge in Periodic Table : The effective nuclear charge of
an atom increases as the number of protons in an atom and across the periodic table as
electrons are held closer to the nucleus due to increase in number of protons and increase in
effective nuclear charge
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



Zeff(F−)=9−2=7+
Zeff(Ne)=10−2=8+
Zeff(Na+)=11−2=9+

So the sodium cation has the greatest effective nuclear charge
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Determination of Effective Nuclear Charge using Slater Rules :


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)
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These do not shield electrons in lower groups
Step 3: Slater's Rules is now broken into two cases:
o the shielding experienced by an s- or p- electron,
 electrons within same group shield 0
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30
 electrons within the n-1 group shield 0
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00
o the shielding experienced by nd or nf valence electrons
 electrons within same group shield 0
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00

Problem : What is the shielding constant experienced by a 2p electron in the nitrogen atom?
Given: Nitrogen (N)
Asked for: S, the shielding constant, for a 2p electron
Method:
A
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B
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Solution A N: 1s2 2s2 2p3
N: (1s2)(2s2,2p3)
Solution B

S[2p] =

0
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35(4)

=

3
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Determine the electron configuration of bromine, then write it in the appropriate form
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Use the appropriate Slater Rule to calculate the shielding constant for the electron
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These do not contribute to the shielding
constant
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00(18) + 0
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15

Problem : What is the effective nuclear charge experienced by a valence p- electron in boron?
Given: Boron (B)
Asked for: Zeff for a valence p- electron
Method:
A
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C
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Determine the electron configuration of boron and identify the electron of interest
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Use the Periodic Table to determine the actual nuclear charge for boron
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Solution:
A) B: 1s2 2s2 2p1
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B: (1s2)(2s2,2p1)
B) S[2p] = 1
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85(2) + 0
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40
C) Z = 5
D) Zeff = Z -  = 2

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