Chemistry 9

1. SHIELDING EFFECT
PERIODIC TABLE AND PERIODICITY
OF PROPERTIES
LESSON NUMBER 7

2. DEFINITION
The electrons present in the
inner shells screen or shield the
force of attraction of nucleus felt
by the valence shell electrons.
This is called shielding effect.
Inner shell electrons are any electrons
not in the outermost shell. They shield
the valence electrons from the nucleus,
reducing the effective nuclear charge.
SHIELDING EFFECT
+
Valence shell
Inner shells

3. EFFECTIVE NUCLEAR CHARGE
SHIELDING EFFECT
The attractions of the outer electrons
towards the nucleus is partially reduced
because of the presence of inner
electrons.
As a result valence electrons experience
less nuclear charge than that of actual
charge, which is called effective nuclear
charge(Zeff).

4. SHIELDING EFFECT
Effective Nuclear Charge
The effective nuclear charge (often symbolized as Zeff ) is
the net positive charge experienced by an electron in a
multi-electron atom.
Zeff = Z – S
Z is the atomic number and S is the number of shielding
electrons
The term “effective” is used because the shielding effect
of negatively charged electrons prevents higher orbital
electrons from experiencing the full nuclear charge.

5. FACTORS AFFECTING THE SHIELDING EFFECT
• An important factor which affects the
magnitude of shielding effect is the
number of inner shell electrons.
• Greater the number of inner shell
electrons, greater will be the magnitude
of shielding effect.
SHIELDING EFFECT

6. Na = 1s22s22p63s1
valence electron=1
inner electrons=10
Cs = 1s22s22p63s23p64s23d104p65s24d105p66s1
Valence electrons=1
Inner electrons = 54
Example

7. TRENDS OF SHIELDING EFFECT IN THE PERIODIC TABLE
ACROSS THE PERIOD
Remain Constant
Reason:
It remains same because the number of
inner shell electrons remains same.

8. Increases
DOWN THE GROUP
TRENDS OF SHIELDING EFFECT IN THE PERIODIC TABLE
Reason:
It increases because the number of inner shells
and inner shell electrons increases down the
group.

9. Potassium, 19K
Rubidium,37Rb
Sodium, 11Na
Lithium, 3Li
DOWN THE GROUP
TRENDS OF SHIELDING EFFECT IN THE PERIODIC TABLE
Cesium, 55Cs

10. SHIELDING EFFECT MAKES CATION FORMATION EASY
• The shielding effect explains why
valence-shell electrons are more
easily removed from the atom.
• Bigger size atoms have more
shielding effect as they are having
more inner shell electrons, so more
easy to remove an electron.

11. Potassium Atom
SHIELDING EFFECT MAKES CATION FORMATION EASY
Potassium Ion
Loses outer
electron
+
Example

12. Which one has more shielding effect?
• Calcium or Magnesium
• Nitrogen or Phosphorus
• Potassium or Rubidium
TEST YOURSELF